Exceptionally unskilled actor (5)
I believe the answer is:
extra
I believe this clue is a double definition.
'exceptionally' is the first definition.
(thesaurus)
'unskilled actor' is the second definition.
(Other definitions for extra that I've seen before include "score (not a hit)" , "Minor actor in crowd scene" , "more than usually" , "Bit-part actor" , "more than required" .)