Unable to perform in Cath's bedroom, when nervous (4,2,6)
I believe the answer is:
hors de combat
'unable to perform' is the definition.
'cath's bedroom when nervous' is the wordplay.
'when nervous' indicates an anagram.
'caths'+'bedroom'='cathsbedroom'
'cathsbedroom' anagrammed gives 'HORS DE COMBAT'.
'in' acts as a link.
(Other definitions for hors de combat that I've seen before include "Injured and out of action (French)" , "Out of action due to injury" , "Disabled, out of the fight" .)