Boys left with a detective sergeant (4)
I believe the answer is:
lads
'boys' is the definition.
(I know this)
'left with a detective sergeant' is the wordplay.
'left' becomes 'l' (common abbreviation).
'with' says to put letters next to each other.
'detective sergeant' becomes 'DS' (abbreviation for detective sergeant).
'l'+'a'+'ds'='LADS'
(Other definitions for lads that I've seen before include "Fellows, chaps" , "Boys, young fellows" , "Young men" , "Fellows, blokes" , "Youngsters" .)