Periodically stroke part of foot (3)
I believe the answer is:
toe
'part of foot' is the definition.
(I know this)
'periodically stroke' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
The alternating letters of 'stroke' are 'TOE'.
(Other definitions for toe that I've seen before include "'Little piggy'" , "One with sock on" , "Part of foot" , "compliantly put on line" , "Drive obliquely" .)