Returning, we had first pen for brother’s little son (6)
I believe the answer is:
nephew
'son' is the definition.
(thesaurus)
'returning we had first pen' is the wordplay.
'returning' shows that the letters should be reversed in order.
'had first' becomes 'h' (1st letter of 'had').
'we'+'h'+'pen'='wehpen'
'wehpen' back-to-front is 'NEPHEW'.
'for brother's little' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for nephew that I've seen before include "A young male relation" , "Family relative" , "Has a duty (to do something)" , "Sibling's son" , "Brother's/sister's son" .)