A regular fixture in the pub - a small measure but only if absolutely necessary!
I believe the answer is:
at a pinch
'if absolutely necessary' is the definition.
(I've seen this before)
'a regular fixture in' is the wordplay.
'regular fixture' becomes 'tap' (tap is a kind of fixture. I am not sure about the 'regular' bit.**).
'in' becomes 'inch' ('inch' can be a synonym of 'in').
'a'+'tap'+'inch'='AT A PINCH'
'the pub a small measure but only' acts as a link.
This may not be correct. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for at a pinch that I've seen before include "in a crisis?" , "in case of emergency" , "under pressure" , "If really necessary" , "if need be" .)