Defines all forms of altruism (4-6)

I believe the answer is:

self-denial

'altruism' is the definition.
(thesaurus)

'defines all forms' is the wordplay.
'forms' indicates an anagram.
'defines'+'all'='definesall'
'definesall' with letters rearranged gives 'SELF-DENIAL'.

'of' acts as a link.

(Another definition for self-denial that I've seen is " Unwillingness to gratify one's own desires".)

I've seen this clue in The Telegraph.

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