Ruin a culex terribly in profuseness (10)
I believe the answer is:
luxuriance
'profuseness' is the definition.
(luxuriance is a kind of profuseness)
'ruin a culex terribly' is the wordplay.
'terribly' indicates anagramming the letters.
'ruin'+'a'+'culex'='ruinaculex'
'ruinaculex' with letters rearranged gives 'LUXURIANCE'.
'in' is the link.
(Other definitions for luxuriance that I've seen before include "Copiousness" , "Lushness" .)