Fuddy-duddy leaving to prepare for a fight (6,3)
I believe the answer is:
square off
'prepare for a fight' is the definition.
(square off can mean to assume a fighting pose)
'fuddy-duddy leaving' is the wordplay.
'fuddy-duddy' becomes 'square' (I can't justify this - if you can you should give a lot more credence to this answer).
'leaving' becomes 'off' (as in 'I'm off').
'square'+'off'='SQUARE OFF'
'to' is the link.
Can you help me to learn more?
(Other definitions for square off that I've seen before include "What boxer might do if attacked" , "prepare for fight" , "Raise dukes" .)