Third child fixed to get husband (4)
I believe the answer is:
seth
'third child' is the definition.
(I've seen this before)
'fixed to get husband' is the wordplay.
'fixed' becomes 'set' (both can mean decided or determined).
'to get' says to put letters next to each other.
'husband' becomes 'h' (abbreviation).
'set'+'h'='SETH'
(Other definitions for seth that I've seen before include "Biblical character" , "Early man" , "Cain/Abel brother" , "Cain and Abel's younger brother" , "Boy" .)