Stage when steroid finally is injected into swine (6)
I believe the answer is:
boards
'stage' is the definition.
(an actor 'treads the boards')
'steroid finally is injected into swine' is the wordplay.
'finally' says to take the final letters.
'is injected into' means one lot of letters goes inside another.
'swine' becomes 'boars' (I have seen 'Wild swine ' mean 'boars' so perhaps 'swine' could also mean 'boars').
The final letter of 'steroid' is 'd'.
'd' going within 'boars' is 'BOARDS'.
'when' acts as a link.
(Other definitions for boards that I've seen before include "Gets on; the stage (colloq.)" , "Theatre stages" , "Actors are said to tread them" , "sections of timber" , "Tread them in the theatre" .)