You can get water at the Boar's Head, one thinks (5)
I believe the answer is:
brain
'head one thinks' is the definition.
'brain' can be an answer for 'head' (head can mean someone's brain or intellect). I am not certain of the 'one thinks' bit.
I can't explain the remainder of the clue.
Can you help me to learn more?
(Other definitions for brain that I've seen before include "Intellectual ability" , "Hit hard; intellect" , "The grey matter" , "Intellectual capacity" , "Thinking thing" .)