Start to describe posh English dance (7)
I believe the answer is:
beguine
'dance' is the definition.
(I've seen this in another clue)
'start to describe posh english' is the wordplay.
'start' becomes 'begin' (synonyms).
'to describe' means one lot of letters goes inside another.
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-U).
'english' becomes 'E' (abbreviation as in OED).
'begin' going around 'u' is 'beguin'.
'beguin'+'e'='BEGUINE'
(Other definitions for beguine that I've seen before include "Dance of French West Indian origin" , "French West Indian 20" , "Cole Porter song: 'Begin the .......'" .)