Thrashing bridge supremo (8)
I believe the answer is:
spanking
'thrashing' is the definition.
(thesaurus)
'bridge supremo' is the wordplay.
'bridge' becomes 'span' ('span' can be a synonym of 'bridge').
'supremo' becomes 'king' (both can refer to a person with great authority).
'span'+'king'='SPANKING'
(Other definitions for spanking that I've seen before include "Quick and energetic (1)" , "procedure for maintaining discipline!" , "Outstanding - traditional punishment for children?" , "Very good" , "Hiding" .)