Stop unpleasant characters going topless in gallery (9)
I believe the answer is:
terminate
'stop' is the definition.
(terminate can mean to stop or bring to an end)
'unpleasant characters going topless in gallery' is the wordplay.
'unpleasant characters' becomes 'vermin' (vermin can mean unpleasant people).
'going topless' means to remove the first letter.
'in' is an insertion indicator.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'vermin' with its initial letter taken away is 'ermin'.
'ermin' inserted inside 'tate' is 'TERMINATE'.
(Other definitions for terminate that I've seen before include "Put an end to, discontinue" , "Cut short" , "Put an end to, finish" , "Bring to an end" , "Kill off" .)