It's handy when your foot's half dead! (4)
I believe the answer is:
deft
'it's handy' is the definition.
(I've seen this before)
'your foot's half dead' is the wordplay.
I cannot really see how this works, but
'dead' could be 'd' (abbreviation in government records) and 'd' is found within the answer.
'foot' could be 'ft' (abbreviation) and 'ft' is located in the answer.
A single letter 'e' remains which might be clued in a way I don't understand.
This explanation may well be incorrect...
'when' is the link.
Can you help me to learn more?
(Other definitions for deft that I've seen before include "Adept" , "Handling things skilfully or neatly" , "Adroit, dexterous" , "Quick and skilful in movement" , "Quick and dexterous" .)