Retro function held in gallery until now (2,4)
I believe the answer is:
to date
'until now' is the definition.
(I've seen this before)
'retro function held in gallery' is the wordplay.
'retro' is a reversal indicator (retro is Latin for 'backwards').
'function' becomes 'do' (I've seen this in other clues).
'held in' indicates putting letters inside.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'do' written backwards gives 'od'.
'od' placed into 'tate' is 'TO DATE'.
(Other definitions for to date that I've seen before include "Until this point in time" , "Up to the present time (2.4)" , "Amorous American's aim," , "Up to this time - check calendar" , "Up until today" .)