Terribly posh dances (4)
I believe the answer is:
hops
'dances' is the definition.
(I know this)
'terribly posh' is the wordplay.
'terribly' indicates an anagram.
'posh' with letters rearranged gives 'HOPS'.
(Other definitions for hops that I've seen before include "what could make you bitter" , "Skips" , "Dances -- beer ingredient" , "Shop around for beer material" , "Beer flavouring" .)