Upsetting Peter, I'm not as full (7)
I believe the answer is:
emptier
'not as full' is the definition.
('empty' can be the opposite of 'full' so perhaps 'emptier' is reasonable for 'not as full')
'upsetting peter i'm' is the wordplay.
'upsetting' indicates an anagram.
'peter'+'im'='peterim'
'peterim' is an anagram of 'EMPTIER'.
(Other definitions for emptier that I've seen before include "Containing less" , "more in need of food" , "I'm Peter (anag)" , "More vacuous" , "Being more hungry" .)