Not just amusement park failing to open (6)
I believe the answer is:
unfair
'not just' is the definition.
('unfair' is the opposite to 'just')
'amusement park failing to open' is the wordplay.
'amusement park' becomes 'funfair' (synonyms).
'failing to open' means to remove the first letter (the opening letter is removed).
'funfair' with its initial letter removed is 'UNFAIR'.
(Other definitions for unfair that I've seen before include "Not just - lacking in beauty" , "not right" , "out of order" , "'Unjust, inequitable (6)'" , "Not equitable or blond" .)