The barber offers to do this for gang from Trim
I believe the answer is:
crew cut
'the barber offers to do this' is the definition.
The definition suggests a singular noun which matches the answer.
'gang from trim' is the wordplay.
'gang' becomes 'crew' ('crew' can be a synonym of 'gang').
'from' means one lot of letters go next to another.
'trim' becomes 'cut' ('cut' can be a synonym of 'trim').
'crew'+'cut'='CREW-CUT'
'for' acts as a link.
Can you help me to learn more?
(Other definitions for crew cut that I've seen before include "Closely cropped hair style for men" , "reduced cover above" , "in which redundancies effected?" , "Outcome of airline economy study" , "top-level style" .)