Break up gang led by detectives (7)
I believe the answer is:
disband
'break up' is the definition.
(disbanding is a kind of breaking up)
'gang led by detectives' is the wordplay.
'gang' becomes 'band' (I've seen this before**).
'led by' means one lot of letters go next to another.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'band' after 'dis' is 'DISBAND'.
(Other definitions for disband that I've seen before include "Scatter" , "Dissolve" , "shed" , "Cease to be a group" , "Break up as a group" .)