Second shopping centre is not big (5)
I believe the answer is:
small
'is not big' is the definition.
('small' is the opposite to 'big')
'second shopping centre' is the wordplay.
'second' becomes 's' (common abbreviation).
'shopping centre' becomes 'mall' (a mall is a large shopping complex).
's'+'mall'='SMALL'
(Other definitions for small that I've seen before include "of little significance" , "Unimportant; little" , "Diminutive" , "Brief" , "Of little size" .)