Not so big (7)
'not so big' is the definition.
('small' can be the opposite of 'big' so perhaps 'smaller' is reasonable for 'not so big')
This is the entire clue.
(Other definitions for smaller that I've seen before include "Sell ram (anag.)" , "Less extensive" , "Not as big as" , "More diminutive" .)
'not so big' is the definition.
(I've seen this in another clue)
This is all the clue.