American assistant upset by pain (6)
I believe the answer is:
apache
'american' is the definition.
(I've seen this before)
'assistant upset by pain' is the wordplay.
'assistant' becomes 'PA' (personal assistant).
'upset' says the letters should be written backwards.
'by' means one lot of letters go next to another.
'pain' becomes 'ache' (ache is a kind of pain).
'pa' backwards is 'ap'.
'ap'+'ache'='APACHE'
(Other definitions for apache that I've seen before include "North American native tribe" , "Geronimo was of this native American tribe" , "member of tribe" , "N American Indian tribe or old Paris ruffian" , "North American Indian tribe - Paris ruffian" .)