Liar must be made to show unselfishness (8)
I believe the answer is:
altruism
'unselfishness' is the definition.
(altruism is a kind of unselfishness)
'liar must be made' is the wordplay.
'be made' is an anagram indicator.
'liar'+'must'='liarmust'
'liarmust' with letters rearranged gives 'ALTRUISM'.
'to show' is the link.
(Other definitions for altruism that I've seen before include "Unselfishness shown by muralist" , "Selflessness" , "Unselfishness, Philanthropy" , "Trial sum (anag.)" , "Unselfish generosity" .)