Order plain clothes officers back to gallery (7)
I believe the answer is:
dictate
'order' is the definition.
(I know that order can be written as dictate)
'plain clothes officers back to gallery' is the wordplay.
'plain clothes officers' becomes 'cid' (I can't explain this - if you can you should believe this answer much more).
'back' shows that the letters should be reversed in order.
'to gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'cid' backwards is 'dic'.
'dic'+'tate'='DICTATE'
Can you help me to learn more?
(Other definitions for dictate that I've seen before include "Lay down the law" , "speak (words to be taken down)" , "Command" , "speak slowly" , "Determine; read out to be taken down" .)