Art gallery hosting revolutionary party until now (2,4)
I believe the answer is:
to date
'until now' is the definition.
(I've seen this in another clue)
'art gallery hosting revolutionary party' is the wordplay.
'art gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'hosting' means one lot of letters goes inside another (inserted letters are welcomed in or hosted).
'revolutionary' is a reversal indicator (letters revolve around).
'party' becomes 'do' (do can informally mean a party).
'do' backwards is 'od'.
'tate' placed around 'od' is 'TO DATE'.
(Other definitions for to date that I've seen before include "Until this point in time" , "Up to this time - check calendar" , "Reason suitor's come" , "Up to the present time (2.4)" , "up to the present day" .)