Exaggerate about gallery pinching original of Sickert (9)
I believe the answer is:
overstate
'exaggerate' is the definition.
(I know that exaggerate can be written as overstate)
'about gallery pinching original of sickert' is the wordplay.
'about' becomes 'over' (both can mean around).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'pinching' is an insertion indicator (taking inside).
'original of' indicates taking the first letters (I've seen 'original' mean this).
The first letter of 'sickert' is 's'.
'over'+'tate'='overtate'
'overtate' going around 's' is 'OVERSTATE'.
(Other definitions for overstate that I've seen before include "Exaggerate, go beyond bounds of truth" , "lay it on thick" , "Exaggerate, make too much of" .)