Bird has foot that's a twelfth longer! (5)
I believe the answer is:
finch
'bird' is the definition.
(finch is a kind of bird)
'foot that's a twelfth longer' is the wordplay.
'foot' becomes 'f' (abbreviation).
'that's a twelfth longer' becomes 'inch' (I am not sure about this - if you are sure you should believe this answer much more).
'f'+'inch'='FINCH'
'has' is the link.
Can you help me to learn more?
(Other definitions for finch that I've seen before include "Type of bird" , "Singer" , "Peter - - , film actor; bird" , "Bird such as canary" , "Songbird such as a canary" .)