The first worker in opposition (4)
I believe the answer is:
anti
'opposition' is the definition.
(I've seen this before)
'the first worker in' is the wordplay.
'first' suggests taking the first letters.
'worker' becomes 'ani' (I am not sure about this - if you are sure you should believe this answer much more).
'in' means one lot of letters goes inside another.
The first letter of 'the' is 't'.
't' inserted within 'ani' is 'ANTI'.
Can you help me to learn more?
(Other definitions for anti that I've seen before include "Prefix meaning against" , "Prefix meaniing against" , "Policy's opponent" , "Naysayer" , "One who is against (something)" .)