Live to become father (5)
I believe the answer is:
beget
'father' is the definition.
(synonyms)
'live to become' is the wordplay.
'live' becomes 'be' (synonyms).
'to become' becomes 'get' (synonyms).
'be'+'get'='BEGET'
(Other definitions for beget that I've seen before include "Engender" , "Sire" , "Give rise to; create (children)" , "Bring about, cause" , "Father" .)