Family member found in outskirts of Newcastle? That's a relief! (6)
I believe the answer is:
Here is my best explanation:
'family member' is the definition.
'outskirts of newcastle? that's a relief' is the subsidiary indication.
'outskirts of' suggests removing the centre.
'that's a relief' becomes 'phew'.
'newcastle' with its centre taken out is 'ne'.
'found in' is the link.
(Other definitions for nephew that I've seen before include "Kinsman", "A relative", "Family relative", "Sibling's son", "kinsman", "A sibling's child", "Brother's/sister's son", "A young male relation", "Relative" and "Male relative".)