Poll pear-tree periodically (4)
I believe the answer is:
pate
'poll' is the definition.
('pate' can be a synonym of 'poll')
'pear-tree periodically' is the wordplay.
'periodically' indicates alternate letters (letters must be selected 'every so often').
'pear'+'tree'='peartree'
The alternate letters of 'peartree' are 'PATE'.
(Other definitions for pate that I've seen before include "Savoury paste; head" , "Posh 8" , "chef's item" , "chef's preparation" , "Tasty meat paste, perhaps foie gras" .)