Unconscious of needing a rest (6)
I believe the answer is:
asleep
'unconscious of' is the definition.
(thesaurus)
'a rest' is the wordplay.
'rest' becomes 'sleep' (sleeping is a kind of resting).
'a'+'sleep'='ASLEEP'
'needing' acts as a link.
(Other definitions for asleep that I've seen before include "this fast?" , "See, pal strangely is not awake" , "In slumber" , "Having forty winks" , "Elapse (anag)" .)