Cavalier not on to help (7)
I believe the answer is:
offhand
'cavalier' is the definition.
(I've seen this before)
'not on to help' is the wordplay.
'not on' becomes 'off' ('off' is the opposite to 'on').
'to help' becomes 'hand' (hand is a kind of help).
'off'+'hand'='OFFHAND'
(Other definitions for offhand that I've seen before include "Ungraciously cool" , "Cavalier" , "Discourteous" , "Casual; unconcerned" , "Casual or curt, like discarded glove?" .)