Plenty backing person that's unpleasant (6)
I believe the answer is:
enough
'plenty' is the definition.
(synonyms)
'backing person that's unpleasant' is the wordplay.
'backing' is a reversal indicator.
'person' becomes 'one' ('one' can mean an unspecified person).
'that's unpleasant' becomes 'ugh' (expression of disgust).
'one' reversed gives 'eno'.
'eno'+'ugh'='ENOUGH'
(Other definitions for enough that I've seen before include "As much as desirable" , "To the required degree" , "The quantity needed, sufficient" , "satisfying" , "Adequate amount to meet the purpose" .)