This young camper’s pants nothing but cosy (3,5)
I believe the answer is:
boy scout
'this young camper's' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'pants nothing but cosy' is the wordplay.
'pants' indicates anagramming the letters (UK informal term for 'nonsense' or 'rubbish').
'nothing' becomes 'o' (looks like zero - 0).
'o'+'but'+'cosy'='obutcosy'
'obutcosy' anagrammed gives 'BOY SCOUT'.
(Other definitions for boy scout that I've seen before include "Baden-Powell follower" , "Woggle-wearing dibber and dobber!" , "Youngster keen to be prepared" , "could be going on sixteen [JAMBOREE]" , "Member of youth organisation, sings around campfires" .)