Set off and left after the other three, we’re told (4,5)
I believe the answer is:
went forth
'the other three we're told' is the definition.
I can't tell whether this definition defines the answer.
'set off and left' is the wordplay.
'set off' becomes 'forth' (I can't explain this - if you can you should believe this answer much more).
'and' says to put letters next to each other (I've seen this in other clues).
'left' becomes 'went' (I've seen this before).
'forth' after 'went' is 'WENT FORTH'.
'after' acts as a link.
Can you help me to learn more?
(Other definitions for went forth that I've seen before include "Departed (arch)" , "Left home, often to do battle" .)