Painter long ago pinching Napoleon's bottom (5)
I believe the answer is:
ernst
'painter' is the definition.
(Max Ernst)
'long ago pinching napoleon's bottom' is the wordplay.
'long ago' becomes 'erst' (archaic term for formerly or long ago).
'pinching' means one lot of letters goes inside another (taking inside).
'bottom' suggests the final letters.
The final letter of 'napoleon' is 'n'.
'erst' enclosing 'n' is 'ERNST'.
(Other definitions for ernst that I've seen before include "German-born painter, Max, co-founder of dadaism, d. 1976" , "Young partner" , "artist's name" , "He painted" , "Max --, German painter" .)