Selflessness that’s really at heart platitude (8)
I believe the answer is:
'selflessness' is the definition.
('altruism' can be a synonym of 'selflessness')
'really at heart platitude' is the wordplay.
'at heart' says to take the centre.
'platitude' becomes 'truism' (I've seen this before).
The middle letters of 'really' are 'al'.
'that's' is the link.
(Other definitions for altruism that I've seen before include "Unselfish action, generosityfor its own sake" , "Pine; last word of Lear" , "Unselfish generosity" , "Unselfishness found in ultraism" , "Selflessness" .)