Boasting about American's support (8)
I believe the answer is:
baluster
'support' is the definition.
(baluster is a kind of support)
'boasting about american's' is the wordplay.
'boasting' becomes 'bluster' (bluster is a kind of boasting).
'about' is an insertion indicator.
'american' becomes 'a' (common abbreviation - e.g. in organisation names).
'bluster' placed around 'a' is 'BALUSTER'.
(Other definitions for baluster that I've seen before include "Pillar supporting staircase rail" , "Type of decorative support for a rail" , "One of a number of closely spaced supports for a railing" , "Pillar supporting a rail, may be rustable" , "This could hold up rail" .)