Nag rest about being peculiar (7)
I believe the answer is:
strange
'peculiar' is the definition.
('strange' can be a synonym of 'peculiar')
'nag rest about' is the wordplay.
'about' is an anagram indicator.
'nag'+'rest'='nagrest'
'nagrest' with letters rearranged gives 'STRANGE'.
'being' is the link.
(Other definitions for strange that I've seen before include "Odd or unknown" , "Queer" , "Irregular" , "Eccentric" , "Weird, eerie" .)