Fertilizer upended in art gallery without right (7)
I believe the answer is:
nitrate
'fertilizer' is the definition.
(I've seen this before)
'upended in art gallery without right' is the wordplay.
'upended' says the letters should be written backwards (in a down clue, letters go up).
'art gallery' becomes 'tate' (Tate Modern, Tate Britain etc).
'without' indicates putting letters inside ('without' can be similar in meaning to 'outside').
'right' becomes 'r' (common abbreviation).
'in' reversed gives 'ni'.
'tate' going around 'r' is 'trate'.
'ni'+'trate'='NITRATE'
(Other definitions for nitrate that I've seen before include "Compound" , "Nattier (anag.)" , "Fertilizer" , "Fertiliser chemical" , "Tear tin up for fertiliser" .)