A bit of ironic humour, mate! (4)
I believe the answer is:
chum
'mate' is the definition.
(I've seen this before)
'a bit of ironic humour' is the wordplay.
'a bit of' indicates a hidden word (I've seen 'a bit' mean this (the answer is formed by a bit of the clue)).
'CHUM' is hidden in the letters of 'ironic humour'.
(Other definitions for chum that I've seen before include "Good friend" , "Close friend (colloq.)" , "Pal, mate" , "Salmon" , "Pal, buddy" .)