Bird exercising with tail cut (5)
I believe the answer is:
pewit
'bird' is the definition.
(pewit is a kind of bird)
'exercising with tail cut' is the wordplay.
'exercising' becomes 'pe' (abbreviation for Physical Education).
'tail cut' means to remove the last letter.
'with' with its final letter taken away is 'wit'.
'pe'+'wit'='PEWIT'
(Other definitions for pewit that I've seen before include "Lapwing" , "Bird" .)