Posh car emerging from pronounced gloom (4)
I believe the answer is:
merc
'posh car' is the definition.
(Mercedes car)
'pronounced gloom' is the wordplay.
'pronounced' indicates a 'sounds like' (homophone) clue.
'gloom' becomes 'murk' (I've seen this before).
'murk' is a homophone of 'MERC'.
'emerging from' is the link.
(Other definitions for merc that I've seen before include "Former cop" , "German car (abbr)" , "High-performance German car" , "business-class car" , "German make of luxury car (colloq.)" .)