Communist, English, ultimately smitten with posh ladies (6)
I believe the answer is:
engels
'communist' is the definition.
(I've seen this before)
'english ultimately smitten with posh ladies' is the wordplay.
'english' becomes 'e' (abbreviation).
'ultimately' indicates one should take the final letters.
'with' says to put letters next to each other.
'posh ladies' becomes 'gels' (gel can mean a posh woman).
The last letter of 'smitten' is 'n'.
'e'+'n'+'gels'='ENGELS'
(Other definitions for engels that I've seen before include "Early Communist philosopher" , "Karl Marx's collaborator" , "German philosopher and collaborator of Marx" , "Marx's co-author" , "Friedrich ---, helped Marx" .)