We're told of fearful worker changing colour (4)
I believe the answer is:
dyer
'worker changing colour' is the definition.
(someone who dyes things)
'we're told of fearful' is the wordplay.
'we're told of' indicates a 'sounds like' (homophone) clue.
'fearful' becomes 'dire' (both can mean terrible).
'dire' sounds like 'DYER'.
(Other definitions for dyer that I've seen before include "Cloth stainer" , "Person colouring cloth" , "Drey (anag.)" , "Adder of colour" , "Colourer (of clothes, perhaps)" .)