Effect backfiring in art gallery thus far (2,4)
I believe the answer is:
to date
'thus far' is the definition.
(I've seen this in another clue)
'effect backfiring in art gallery' is the wordplay.
'effect' becomes 'do' (synonyms).
'backfiring' says the letters should be written backwards.
'in' is an insertion indicator.
'art gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'do' written backwards gives 'od'.
'od' going into 'tate' is 'TO DATE'.
(Other definitions for to date that I've seen before include "up to the present day" , "Amorous American's aim," , "Thus far" , "Until this point in time" , "Reason suitor's come" .)