Actors in work must be robust (4-4)
I believe the answer is:
cast-iron
'robust' is the definition.
('cast-iron' can be similar in meaning to 'robust')
'actors in work' is the wordplay.
'work' indicates anagramming the letters.
'actors'+'in'='actorsin'
'actorsin' with letters rearranged gives 'CAST-IRON'.
'must be' is the link.
(Other definitions for cast-iron that I've seen before include "Very strong" , "Firm and unchangeable" , "Extremely robust" , "Unbreakable" , "Rigid, unyielding" .)