Mark's following upset American bears (8)
I believe the answer is:
sustains
'bears' is the definition.
(I've seen this before)
'mark's following upset american' is the wordplay.
'mark's' becomes 'stains' ('stain' can be a synonym of 'mark').
'following' means one lot of letters go next to another.
'upset' says the letters should be written backwards.
'american' becomes 'us' (abbreviation for United States).
'us' in reverse letter order is 'su'.
'stains' after 'su' is 'SUSTAINS'.
(Other definitions for sustains that I've seen before include "It's Susan (anag.)" , "Nourishes or bolsters" , "Bears" , "Supports, keeps alive" , "Supports physically or mentally" .)