Condiment with French wine base put on fish (7)
I believe the answer is:
vinegar
'condiment' is the definition.
(vinegar is a kind of condiment)
'french wine base put on fish' is the wordplay.
'french wine' becomes 'vin' ('wine' in French).
'base' becomes 'e' (mathematical symbol).
'put on' means one lot of letters go next to another (some letters go on or after others).
'fish' becomes 'gar' (type of fish).
'vin'+'e'+'gar'='VINEGAR'
'with' acts as a link.
(Other definitions for vinegar that I've seen before include "Ill-temper" , "Pickling agent" , "Sour-tasting liquid as condiment" , "Pickling fluid" , "Condiment - bad temper" .)