Abba song mostly average (3)
I believe the answer is:
sos
'abba song' is the definition.
(song by ABBA)
'mostly average' is the wordplay.
'mostly' means to remove the last letter.
'average' becomes 'so-so' (I've seen this before).
'soso' with its last letter taken away is 'SOS'.
(Other definitions for sos that I've seen before include "Help initially, Mr Morse" , "International distress code" , "Call for help" , "request for aid" , "Mayday call" .)