Old duke in gallery until now (2,4)
I believe the answer is:
to date
'until now' is the definition.
(I've seen this before)
'old duke in gallery' is the wordplay.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'duke' becomes 'd' (abbreviation).
'in' means one lot of letters goes inside another.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'o'+'d'='od'
'od' inserted inside 'tate' is 'TO DATE'.
(Other definitions for to date that I've seen before include "Reason suitor's come" , "Up to this time - check calendar" , "Up to the present time (2.4)" , "Until this point in time" , "Up until today" .)