Source of light to cry over (4)
I believe the answer is:
bulb
'source of light' is the definition.
'cry over' is the wordplay.
'cry' becomes 'blub' (I've seen this before).
'over' says the letters should be written backwards.
'blub' reversed gives 'BULB'.
'to' acts as a link.
(Other definitions for bulb that I've seen before include "Reproductive organ in some plants" , "Corm; electric light" , "Tulip, for example" , "A tulip grows from one" , "Source of flower or light" .)