On once more, second time (7)
I believe the answer is:
against
'on' is the definition.
(thesaurus)
'once more second time' is the wordplay.
'once more' becomes 'again' ('again' can be a synonym of 'once more').
'second' becomes 's' (common abbreviation).
'time' becomes 't'.
'again'+'s'+'t'='AGAINST'
(Other definitions for against that I've seen before include "Opposed" , "Not favouring" , "close by" , "Opposite to" , "Abutting" .)