Write down music: number above gallery (6)
I believe the answer is:
notate
'write down music' is the definition.
(as in music notation)
'number above gallery' is the wordplay.
'number' becomes 'no' (abbreviation).
'above' means one lot of letters go next to another (in a down clue, some letters go above others).
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'no'+'tate'='NOTATE'
(Other definitions for notate that I've seen before include "Add runs to score, possibly" , "Record in bars" , "Write in system of signs or symbols" , "At Eton (anag.)" , "Put down in symbols" .)