The first man to be a mother (4)
I believe the answer is:
adam
'the 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'
'to be' acts as a link.
(Other definitions for adam that I've seen before include "A mad sort of ancestor" , "God's first son" , "father of us all?" , "one might confuse with total stranger" , "__ Smith, author of The Wealth of Nations" .)