Decadent English fellows having summer in Paris (6)
I believe the answer is:
effete
'decadent' is the definition.
('effete' can be a synonym of 'decadent')
'english fellows having summer in paris' is the wordplay.
'english' becomes 'e' (abbreviation).
'fellows' becomes 'ff'.
'having' says to put letters next to each other.
'summer in paris' becomes 'ete' (').
'e'+'ff'+'ete'='EFFETE'
(Other definitions for effete that I've seen before include "Over-refined, exhausted" , "Affected, ineffectual" , "Lacking in power" , "Over-refined and ineffectual" , "Over-refined, pretentious" .)