First guy to be a mother (4)
I believe the answer is:
adam
'first guy' is the definition.
(I've seen this before)
'a mother' is the wordplay.
'mother' becomes 'dam' (female parent of an animal).
'a'+'dam'='ADAM'
'to be' acts as a link.
(Other definitions for adam that I've seen before include "According to the Bible, progenitor of the human race" , "God's first son" , "Biblical first man" , "one might confuse with total stranger" , "- - Bede by George Eliot" .)