Head left periodically for a swift one? (4)
I believe the answer is:
half
'a swift one?' is the definition.
(both can mean a quick or small alcoholic drink)
'head left periodically' is the wordplay.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'head'+'left'='headleft'
The alternate letters of 'headleft' are 'HALF'.
'for' is the link.
(Other definitions for half that I've seen before include "Fifty percent" , "One of two equal parts" , "little drink" , "Some football" , "shared round" .)