Company actor's top covered by plain item of clothing (8)
I believe the answer is:
overcoat
'item of clothing' is the definition.
(overcoat is a type of item of clothing)
'company actor's top covered by plain' is the wordplay.
'company' becomes 'co' (abbreviation).
'top' indicates taking the first letters.
'covered by' indicates putting letters inside.
'plain' becomes 'overt' (I've seen this in other clues).
The initial letter of 'actors' is 'a'.
'co'+'a'='coa'
'coa' put into 'overt' is 'OVERCOAT'.
(Other definitions for overcoat that I've seen before include "Evocator (anag.)" , "Inverness" , "Heavy outer garment" , "warmer" , "Outerwear" .)