It's not just one-sided? (6)
I believe the answer is:
unfair
'one-sided?' is the definition.
(similar in meaning)
'it's not just' is the wordplay.
'it's not' becomes 'un' (prefix meaning 'not').
'just' becomes 'fair' (just can mean fair in the sense of showing justice).
'un'+'fair'='UNFAIR'
(Other definitions for unfair that I've seen before include "Dirty" , "Weighted" , "'Unjust, inequitable (6)'" , "raining perhaps?" , "Not equitable, biased" .)