Bird's limb digs every so often (4)
I believe the answer is:
ibis
'bird's' is the definition.
(ibis is a kind of bird)
'limb digs every so often' is the wordplay.
'every so often' indicates alternate letters.
'limb'+'digs'='limbdigs'
The alternate letters of 'limbdigs' are 'IBIS'.
(Other definitions for ibis that I've seen before include "Type of bird" , "Glossy wading bird" , "Bird venerated by ancient Egyptians" , "Slowly discharge" , "Wading bird of warm regions" .)