Impose detectives around gallery (7)
I believe the answer is:
dictate
'impose' is the definition.
(I know that dictate is a more specific form of the action impose)
'detectives around gallery' is the wordplay.
'detectives' becomes 'CID' (abbreviation for Criminal Investigation Department).
'around' says the letters should be written backwards.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'cid' back-to-front is 'dic'.
'dic'+'tate'='DICTATE'
(Other definitions for dictate that I've seen before include "Command" , "say" , "speak (words to be taken down)" , "Lay down the law" , "Issue commands" .)