Girl presented by one’s second son (6)
I believe the answer is:
isabel
'girl' is the definition.
'by one's second son' is the wordplay.
'by' means one lot of letters go next to another.
'one's' becomes 'is' (roman numeral).
'second son' becomes 'abel' (I've seen this before).
'is'+'abel' is 'ISABEL'.
'presented' acts as a link.
(Other definitions for isabel that I've seen before include "Abseil with confused girl" , "Drab" , "Miss" , "Girl's name" , "- Allende, Chilean novelist" .)