Justifies why they might be parsimonious with the beer (9)
I believe the answer is:
rationale
'justifies why they might' is the definition.
The definition suggests an adverb but the answer is not.
'parsimonious with the beer' is the wordplay.
'parsimonious' becomes 'ration' (I can't explain this - if you can you should give a lot more credence to this answer).
'with' means one lot of letters go next to another.
'the beer' becomes 'ale' (type of beer).
'ration'+'ale'='RATIONALE'
'be' is the link.
Can you help me to learn more?
(Other definitions for rationale that I've seen before include "Underlying principle" , "Motivation" , "grounds?" , "Philosophy" , "The logical basis of some thing or action" .)