A grim son, weird life form! (8)
I believe the answer is:
organism
'life form' is the definition.
('organism' can be a synonym of 'life form')
'a grim son weird' is the wordplay.
'weird' indicates anagramming the letters.
'a'+'grim'+'son'='agrimson'
'agrimson' anagrammed gives 'ORGANISM'.
(Other definitions for organism that I've seen before include "Individual life form" , "individual creation" , "Any living entity" , "Body - cell - Mao grins (anag)" , "Something alive" .)