Work fast to impress posh lush (7)
I believe the answer is:
opulent
'lush' is the definition.
(thesaurus)
'work fast to impress posh' is the wordplay.
'work' becomes 'op' (abbreviation for opus).
'fast' becomes 'lent' (I've seen this before).
'to impress' means one lot of letters goes inside another (impress can mean enlist or recruit).
'posh' becomes 'u' (abbreviation for upper-class - opposite to non-u).
'op'+'lent'='oplent'
'oplent' going around 'u' is 'OPULENT'.
(Other definitions for opulent that I've seen before include "'Loaded'" , "Let up on, being wealthy" , "No let-up (anag)" , "indicating wealth" , "Ritzy" .)