Big chaps set about ending in various theatre roles (9)

'big chaps' is the definition.
(informal term for a tall person)

'set about ending in various theatre roles' is the wordplay.
'set about' shows that the letters should be reversed in order.
'ending in' indicates one should take the final letters.
'theatre' becomes 'rep' (I've seen this in other clues).
'roles' becomes 'parts' ('part' can be a synonym of 'role').
The final letter of 'various' is 's'.
's'+'rep'+'parts'='srepparts'
'srepparts' back-to-front is 'STRAPPERS'.