Philanthropy is a liberal axiom (8)
I believe the answer is:
altruism
'philanthropy' is the definition.
(thesaurus)
'a liberal axiom' is the wordplay.
'a liberal' becomes 'al' (I've seen this in other clues).
'axiom' becomes 'truism' (I can't explain this - if you can you should believe this answer much more).
'al'+'truism'='ALTRUISM'
'is' is the link.
Can you help me to learn more?
(Other definitions for altruism that I've seen before include "Unselfishness found in ultraism" , "Consideration for others first" , "Trial sum (anag.)" , "Unselfish concern for the welfare of others" , "Unselfish generosity" .)