Gallery containing too much so far (2,4)
I believe the answer is:
to date
'so far' is the definition.
(I've seen this before)
'gallery containing too much' is the wordplay.
'gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'containing' indicates putting letters inside.
'too much' becomes 'od' (abbreviation for overdose).
'tate' going around 'od' is 'TO DATE'.
(Other definitions for to date that I've seen before include "Reason suitor's come" , "up to the present day" , "Up until today" , "Amorous American's aim," , "Thus far" .)