Bird's shrewdness on turning tail (6)
I believe the answer is:
godwit
'bird's' is the definition.
(godwit is a kind of bird)
'shrewdness on turning tail' is the wordplay.
'shrewdness' becomes 'wit' (both can mean cleverness).
'on' means one lot of letters go next to another.
'turning' shows that the letters should be reversed in order.
'tail' becomes 'dog' (both can mean to follow).
'dog' back-to-front is 'god'.
'wit' put after 'god' is 'GODWIT'.
(Other definitions for godwit that I've seen before include "Wader; two dig (anag.)" , "Long-billed wader" , "Wader; do twig (anag.)" , "Flyer" , "Large wading bird" .)