Force clown to entertain American zealot (7)
I believe the answer is:
fanatic
'zealot' is the definition.
(thesaurus)
'force clown to entertain american' is the wordplay.
'force' becomes 'f' (symbol used in Physics e.g. f=ma).
'clown' becomes 'antic' ('antic' can be a synonym of 'clown').
'to entertain' indicates putting letters inside.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'antic' placed around 'a' is 'anatic'.
'f'+'anatic'='FANATIC'
(Other definitions for fanatic that I've seen before include "Narrow-minded enthusiast" , "Freak" , "Extreme enthusiast" , "enthusiastic" , "Person filled with excessive zeal for cause" .)