Awful hunger -- it keeps American more mischievous (9)
I believe the answer is:
naughtier
'more mischievous' is the definition.
('naughty' can be a synonym of 'mischievous')
'awful hunger it keeps american' is the wordplay.
'awful' is an anagram indicator.
'keeps' is an insertion indicator (inserted letters are kept inside).
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'hunger'+'it'='hungerit'
'hungerit' anagrammed gives 'nughtier'.
'nughtier' placed around 'a' is 'NAUGHTIER'.
(Other definitions for naughtier that I've seen before include "Less well behaved" , "More mischievous" , "more unruly" , "More badly behaved" , "Worse" .)