Pen returned, he started walking with a younger relative (6)
I believe the answer is:
nephew
'a younger relative' is the definition.
(the son of a sibling)
'pen returned he started walking' is the wordplay.
'returned' shows that the letters should be reversed in order.
'started' says to take the initial letters.
The first letter of 'walking' is 'w'.
'pen' reversed gives 'nep'.
'nep'+'he'+'w'='NEPHEW'
'with' acts as a link.
(Other definitions for nephew that I've seen before include "Edward V to Richard III relation" , "Male relative" , "Family relative" , "Has a duty (to do something)" , "Brother's/sister's son" .)