One son, Nigel: he's most peculiar (6)
I believe the answer is:
single
'one' is the definition.
(synonyms)
'son nigel he's most peculiar' is the wordplay.
I cannot really see how this works, but
'son' could be 's' (genealogical abbreviation for son) and 's' is present in the answer.
an anagram of 'nigel' is 'ingle' which is within the answer.
No letters remain.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for single that I've seen before include "Unaccompanied" , "Not in a relationship" , "Solo" , "Free" , "Like bachelor" .)