"SOS" pop group (4)
I believe the answer is:
abba
'sos pop group' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for abba that I've seen before include "Biblical pop" , "Group - father" , "rhymes in Petrarchan sonnet" , "Coptic bishop's title; Swedish pop group" , "Waterloo singers" .)