Mark’s going around the United States with supports (8)
I believe the answer is:
sustains
'supports' is the definition.
(I know this)
'mark's going around the united states' is the wordplay.
'mark's' becomes 'stains' ('stain' can be a synonym of 'mark').
'going around' is an insertion indicator.
'the united states' becomes 'U.S.' (abbreviation for the United States).
'stains' going around 'us' is 'SUSTAINS'.
'with' acts as a link.
(Other definitions for sustains that I've seen before include "keeps going" , "Nourishes or bolsters" , "Provides with nourishment" , "It's Susan (anag.)" , "Supports physically or mentally" .)