Cry over an onion? (4)
I believe the answer is:
bulb
'an onion?' is the definition.
(onion is a kind of bulb)
'cry over' is the wordplay.
'cry' becomes 'blub' (I've seen this before).
'over' shows that the letters should be reversed in order.
'blub' in reverse letter order is 'BULB'.
(Other definitions for bulb that I've seen before include "shiner" , "Underground storage organ for some plants" , "Onion, possibly" , "Eg, hyacinth" , "Onion, perhaps" .)