Peter is upset wanting relief (7)
I believe the answer is:
respite
'relief' is the definition.
(I know that relief can be written as respite)
'peter is upset' is the wordplay.
'upset' indicates anagramming the letters.
'peter'+'is'='peteris'
'peteris' anagrammed gives 'RESPITE'.
'wanting' acts as a link.
(Other definitions for respite that I've seen before include "Short break" , "Short let-up" , "Temporary break" , "Abatement, rest" , "Brief let-up" .)