Prime Minister swallowing litres for physical well-being (6)
I believe the answer is:
health
'physical well-being' is the definition.
(I've seen this before)
'prime minister swallowing litres' is the wordplay.
'prime minister' becomes 'heath' (Edward Heath).
'swallowing' indicates putting letters inside.
'litres' becomes 'L' (abbreviation).
'heath' going around 'l' is 'HEALTH'.
'for' is the link.
(Other definitions for health that I've seen before include "Fettle -- condition" , "Good condition" , "good shape" , "It's said to be more than wealth" , "Fit condition" .)