Any living thing with heart, for one, is master (8)
I believe the answer is:
organism
'any living thing' is the definition.
(I've seen this before)
'with heart for one is master' is the wordplay.
'with' is a charade indicator (letters next to each other).
'heart for one' becomes 'organ' (heart is a kind of organ**).
'master' becomes 'm'.
'organ' put next to 'is' is 'organis'.
'organis'+'m'='ORGANISM'
(Other definitions for organism that I've seen before include "Living animal or vegetable" , "Something alive" , "Body - cell - Mao grins (anag)" , "Any living entity" , "Living thing -- man's giro (anag)" .)