Whether he's mostly mother or monster ... (8)
I believe the answer is:
behemoth
'monster' is the definition.
(I know that monster can be written as behemoth)
'whether he's mostly mother' is the wordplay.
I cannot really understand how this works, but
'he' could be 'em' and 'em' is found within the answer.
This may be the basis of the clue (or it may be nonsense).
'or' is the link.
Can you help me to learn more?
(Other definitions for behemoth that I've seen before include "Monstrous beast (Job)" , "Anything abnormally large and powerful" , "Abnormally large creature" , "Big corporation" , "Huge organisation" .)