Actor portrays overwhelming lethargy (6)
I believe the answer is:
torpor
'lethargy' is the definition.
(I've seen this before)
'actor portrays overwhelming' is the wordplay.
'overwhelming' indicates a hidden word (the clues covers up or 'overwhelms' the answer).
'TORPOR' is hidden within 'actor portrays'.
(Other definitions for torpor that I've seen before include "lack of energy" , "Numbness - inactivity" , "Listlessness" , "State of sluggish somnolence" , "Inactive and sluggish condition" .)