When one’s all set, it goes into action (9)

'it goes into action' is the definition.
I don't know anything about this answer so I cannot judge whether this works.

'when one's all set' is the wordplay.
I cannot really understand how this works, but
'when' could be 'e' (I've seen this in other clues) and 'e' is found within the answer.
'one' could be 'i' (Roman numeral) and 'i' is found in the answer.
'set' could be 'hard' ('hard' can be similar in meaning to 'set') and 'hard' is found within the remaining letters.
This may be the basis of clue (or it may be nonsense).

Can you help me to learn more?