South arcade is not big (5)
I believe the answer is:
small
'is not big' is the definition.
('small' is the opposite to 'big')
'south arcade' is the wordplay.
'south' becomes 'S' (synonyms).
'arcade' becomes 'mall' (I've seen this in another clue).
's'+'mall'='SMALL'
(Other definitions for small that I've seen before include "Trifle -- half a pint?" , "Diminutive" , "Social chat" , "Young; modest" , "Piddling" .)