Complain when bloke brought in omicron (4)
I believe the answer is:
moan
'complain' is the definition.
(moan can mean to complain or whinge)
'bloke brought in omicron' is the wordplay.
'bloke' becomes 'man' (informal term for a man).
'brought in' is an insertion indicator.
'omicron' becomes 'o' (this might be a standard abbreviation of which I'm unaware).
'man' enclosing 'o' is 'MOAN'.
'when' is the link.
Can you help me to learn more?
(Other definitions for moan that I've seen before include "Lament; grumble" , "Complaint" , "Complain mournfully" , "Indicate pain" , "Complain, grumble" .)