Worker, one not pro (4)
I believe the answer is:
anti
'not pro' is the definition.
('anti' is the opposite to 'pro')
'worker one' is the wordplay.
'worker' becomes 'ant' (worker is a type of ant).
'one' becomes 'i' (Roman numeral).
'ant'+'i'='ANTI'
(Other definitions for anti that I've seen before include "Adversary" , "Opposition member" , "One who is against (something)" , "Naysayer" , "Who pooh-poohs all" .)