A mother's boy (4)
I believe the answer is:
adam
'boy' is the definition.
'a mother's' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'a'+'dam'='ADAM'
(Other definitions for adam that I've seen before include "Biblical first human" , "Man No. 1" , "One of our common ancestors?" , "Man on the ceiling of the Sistine Chapel" , "Faith, for example" .)