Fix weapon held by American, protesting hotly (2,2,4)
I believe the answer is:
up in arms
'protesting hotly' is the definition.
(I've seen this before)
'fix weapon held by american' is the wordplay.
'fix' becomes 'pin' (to pin is to fix in place).
'weapon' becomes 'arm' (synonyms).
'held by' means one lot of letters goes inside another.
'american' becomes 'US' (abbreviation for United States).
'pin'+'arm'='pinarm'
'pinarm' going within 'us' is 'UP IN ARMS'.
(Other definitions for up in arms that I've seen before include "Better prepared for battle" , "Rioting" , "being angry" , "Protesting vigorously, ready for fight" , "hot under the collar" .)