It's oddly naive to convene outside with a relation (5)
I believe the answer is:
niece
'a relation' is the definition.
(I know that relation can be written as niece)
'it's oddly naive to convene outside' is the wordplay.
'it's oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
'outside' means to remove the middle letters.
'convene' with its middle removed is 'ce'.
The alternate letters of 'naive' are 'nie'.
'nie'+'ce'='NIECE'
'with' is the link.
(Other definitions for niece that I've seen before include "member of different generation" , "Female relation" , "girl further down the line" , "Family relation" , "Brother's daughter" .)