Introducing gallery, sign marked (7)
I believe the answer is:
notated
'marked' is the definition.
(I've seen this before)
'introducing gallery sign' is the wordplay.
'introducing' indicates putting letters inside (some letters introduced or inserted into others).
'gallery' becomes 'tate' (I've seen this before).
'sign' becomes 'nod' (I have seen 'Sign of assent' mean 'nod' so perhaps 'sign' could also mean 'nod').
'tate' inserted within 'nod' is 'NOTATED'.
(Other definitions for notated that I've seen before include "wrote music?" , "Marked" , "symbolically portrayed" , "Set down in symbols" , "commented on" .)