Your setter's handy but won't work (11)
I believe the answer is:
impractical
'won't work' is the definition.
I can't tell whether this defines the answer.
'your setter's handy' is the wordplay.
'your setter's' becomes 'I'm' ('I'm', referring to the setter of the crossword).
'handy' becomes 'practical' (handy can mean practical or convenient).
'im'+'practical'='IMPRACTICAL'
'but' is the link.
Can you help me to learn more?
(Other definitions for impractical that I've seen before include "That's not much use" , "Unworkable" , "Not sensible" , "Unrealistic" .)