Time to start fires (4,2)
I believe the answer is:
'fires' is the definition.
(I've seen this before)
'time to start' is the wordplay.
'time' becomes 'lets' (I can't justify this - if you can you should give a lot more credence to this answer).
'to start' becomes 'go' ('go' can be a synonym of 'start').
Can you help me to learn more?
(Other definitions for lets go that I've seen before include "Liberates" , "fires" , "Order to leave" , "Releases from one's grip" , "Come on then, we're off!" .)