Not by RAW: "bright light" =/= "sunlight" =/= "The Sun"
Magnifying Glass. Lighting a fire with a magnifying glass requires light as bright as sunlight to focus, tinder to ignite, and about 5 minutes for the fire to ignite. Basic Rules, p. 51)
None of the rules indicate heat as a characteristic of the various light spells, although some "are sunlight" and can do radiant damage. The light cantrip is not described as being sunlight, nor "as bright as sunlight." Light emitting spells that might suffice:
8th-level evocation / Duration: Instantaneous
Brilliant sunlight flashes in a 60-foot radius centered on a point you choose within range.
That spell doesn't last 5 minutes.
3rd level / Duration: 1 hour
A 60 foot radius sphere of light spreads out from a point you choose within range. The sphere is bright light and sheds dim light for an additional 60 feet.
It lasts long enough, but isn't sunlight.
Cantrip / Duration: 1 hour
You touch one object that is no larger than 10 feet in any dimension. Until the spell ends, the object sheds bright light in a 20-foot radius and dim light for an additional 20 feet.
Lasts long enough, but it isn't sunlight.
5th level / Duration: Concentration, up to 1 minute
The light of dawn shines down on a location you specify within range. Until the spell ends, a 30-foot-radius, 40-foot-high cylinder of bright light glimmers there. This light is sunlight.
Yes sunlight, not a 5 minute duration.
Note: Whether or not radiant damage includes "heat" or is some other kind of magical / holy radiation is unclear. Not all light sources can do what the sun does.
- For example, my LED flashlight illuminates our kitchen pretty well when I hang it from the rafters, but I can't concentrate that light into something that will start a fire. Yes, I tried on a challenge from my son.
Is this a cool idea for lighting a fire? Yes.
Under Rules as Fun, you could rule as a DM that with a bit of time and effort, a cleric could (with a suitable magnifying glass, or with a similar item like a lens from a spyglass (Expensive!)) concentrate enough heat from the light into something that starts a fire. But a tinderbox is a lot cheaper. Do you want to dismantle that spyglass, and perhaps not be able to use it?
Magnifying glass...100 gp
(Basic Rules, p. 50)
There are other cantrips that can light a fire as part of their basic function: druidcraft, produce flame, or that old standby prestidigitation
• You instantaneously light or snuff out a candle, a torch, or a small campfire. (Basic Rules, p. 102)