Charmer backing winner, I seethe somewhat (5)
I believe the answer is:
siren
'charmer' is the definition.
(I've seen this before)
'backing winner i seethe somewhat' is the wordplay.
'backing' is a reversal indicator.
'somewhat' indicates a hidden word.
'neris' is hidden in the letters of 'winner i seethe'.
'neris' written backwards gives 'SIREN'.
(Other definitions for siren that I've seen before include "Dangerously alluring woman" , "Dangerously fascinating woman" , "Device making a loud wailing sound" , "Loud whistle or dangerous woman" , "Warning device, alluring woman" .)