One of the first men to have a unit (4)
I believe the answer is:
abel
'one of the first men' is the definition.
(son of Adam and Eve)
'a unit' is the wordplay.
'unit' becomes 'bel' (bel is a kind of unit).
'a'+'bel'='ABEL'
'to have' is the link.
(Other definitions for abel that I've seen before include "Slain biblical brother" , "Son of Adam and Eve" , "Cain's younger brother" , "Bale out with murder victim" , "Adam's boy" .)