Tart, small, for us (4)
I believe the answer is:
sour
'tart' is the definition.
('sour' can be similar in meaning to 'tart')
'small for us' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'for us' becomes 'our' (I've seen this in other clues).
's'+'our'='SOUR'
(Other definitions for sour that I've seen before include "Having an acidic taste" , "Turned - morose" , "Acrid or sullen" , "Having an acid taste or smell" , "Rancid or tart" .)