This setter's gallery includes one copy (7)
I believe the answer is:
imitate
'copy' is the definition.
(I know that copy can be written as imitate)
'this setter's gallery includes one' is the wordplay.
'this setter's' becomes 'im' (I am - referring to the crossword setter).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'includes' is an insertion indicator.
'one' becomes 'i' (Roman numeral).
'im'+'tate'='imtate'
'imtate' placed around 'i' is 'IMITATE'.
(Other definitions for imitate that I've seen before include "Take as a model" , "I ate Tim! (anag.)" , "Mimic or follow as a model" , "'Ape, copy (7)'" , "I tame it to copy it" .)