Stop a party I backed (6)
I believe the answer is:
period
'stop' is the definition.
(I know that stop can be written as period)
'a party i backed' is the wordplay.
'a' becomes 'per' (eg 'one a day' means 'one per day').
'party' becomes 'do' (do is a kind of party).
'backed' says the letters should be written in reverse.
'do'+'i'='doi'
'doi' reversed gives 'iod'.
'per'+'iod'='PERIOD'
(Other definitions for period that I've seen before include "Span of time" , "term" , "Phase" , "Stop in America" , "Time allocated for lesson in school" .)