Has to confine pullets, every so often, which is unfortunate (7)
I believe the answer is:
hapless
'is unfortunate' is the definition.
(similar in meaning)
'has to confine pullets every so often' is the wordplay.
'to confine' is an insertion indicator.
'every so often' means one should take alternating letters.
The alternate letters of 'pullets' are 'ples'.
'has' placed around 'ples' is 'HAPLESS'.
'which' is the link.
(Other definitions for hapless that I've seen before include "Piteous" , "Ill-starred" , "He slaps (anag) - unlucky" , "Unlucky, unfortunate" , "Pathetic" .)