The thing badly placed in gallery to amuse (9)
I believe the answer is:
titillate
'amuse' is the definition.
(thesaurus)
'the thing badly placed in gallery' is the wordplay.
'the thing' becomes 'it'.
'badly' becomes 'ill' ('ill' can be adverbial as in 'ill-advised').
'placed in' is an insertion indicator (I've seen this in other clues).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'it'+'ill'='itill'
'itill' inserted inside 'tate' is 'TITILLATE'.
'to' acts as a link.
(Other definitions for titillate that I've seen before include "Thrill" , "Excite pleasurably" , "Excite erotically" , "Stimulate agreeably" , "Pique" .)