Source of light to cry over (4)
I believe the answer is:
Here is my best explanation:
'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 in reverse.
'blub' written backwards gives 'BULB'.
'to' is the link.
(Other definitions for bulb that I've seen before include "A tulip grows from one", "Corm; electric light", "This provides light", "Source of flower or light", "Reproductive organ in some plants", "Onion, perhaps", "Eg, onion", "From which a tulip or light comes", "shiner" and "Growing source".)