Old bit of French bread right for tart (4)
I believe the answer is:
sour
'tart' is the definition.
('sour' can be similar in meaning to 'tart')
'old bit of french bread right' is the wordplay.
'old bit of french bread' becomes 'sou' (historical French coin -'bread' is slang for money).
'right' becomes 'r' (common abbreviation).
'sou'+'r'='SOUR'
'for' acts as a link.
(Other definitions for sour that I've seen before include "Vinegary" , "Curdled" , "Acetic" , "Acid tasting" , "Having an acid taste or smell" .)