Detectives are liable for stretch (7)
I believe the answer is:
distend
'stretch' is the definition.
(I've seen this before)
'detectives are liable' is the wordplay.
'detectives' becomes 'dis' (Detective Inspectors, DIs).
'are liable' becomes 'tend' (to tend to do something is to be liable to do it).
'dis'+'tend'='DISTEND'
'for' is the link.
(Other definitions for distend that I've seen before include "Fill out" , "balloon" , "Stretch" , "Cause to swell" , "Expand, inflate" .)