First man could be a mother! (4)
I believe the answer is:
adam
'first man' is the definition.
(first man in the Bible)
'a mother' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'a'+'dam'='ADAM'
'could be' is the link.
(Other definitions for adam that I've seen before include "Early man" , "God's first son" , "Biblical progenitor of human race" , "Biblical first man" , "Robert -, Sc. architect" .)