Second arcade is not big (5)
I believe the answer is:
small
'is not big' is the definition.
('small' is the opposite to 'big')
'second arcade' is the wordplay.
'second' becomes 's' (common abbreviation).
'arcade' becomes 'mall'.
's'+'mall'='SMALL'
(Other definitions for small that I've seen before include "Young; modest" , "Of no great size" , "Limited in size or scope" , "Undersized" , "Not large" .)