Incomplete drawing -- last in gallery (7)
I believe the answer is:
sketchy
'incomplete' is the definition.
('sketchy' can be similar in meaning to 'incomplete')
'drawing last in gallery' is the wordplay.
'drawing' becomes 'sketch' (sketch is a kind of drawing).
'last in' suggests the final letters.
The last letter of 'gallery' is 'y'.
'sketch'+'y'='SKETCHY'
(Other definitions for sketchy that I've seen before include "Incomplete; rough" , "Imperfect" , "lacking detail" , "Superficial" , "Not thorough or detailed" .)