Small and fat (6)
I believe the answer is:
stubby
'small and fat' is the definition.
I can't tell whether this defines the answer.
'small and fat' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'and' means one lot of letters go next to another.
'fat' becomes 'tubby' (similar in meaning).
's'+'tubby'='STUBBY'
(Other definitions for stubby that I've seen before include "Short and blunt" , "Squat" , "Short, thick and strong" , "Thickset" .)