Mark welcoming American support (7)
I believe the answer is:
sustain
'support' is the definition.
(I know that support can be written as sustain)
'mark welcoming american' is the wordplay.
'mark' becomes 'stain' (synonyms).
'welcoming' is an insertion indicator (in sense of 'inviting in').
'american' becomes 'us' (abbreviation for United States).
'stain' placed around 'us' is 'SUSTAIN'.
(Other definitions for sustain that I've seen before include "Feed, nourish" , "Undergo" , "Maintain" , "Bear (something's weight)" , "Nurture, uphold" .)