...support American capital in boasting (8)
I believe the answer is:
baluster
'support' is the definition.
(baluster is a kind of support)
'american capital in boasting' is the wordplay.
'capital' says to take the initial letters.
'in' means one lot of letters goes inside another.
'boasting' becomes 'bluster' (bluster is a kind of boasting).
The initial letter of 'american' is 'a'.
'a' put within 'bluster' is 'BALUSTER'.
(Other definitions for baluster that I've seen before include "Set of posts supporting a rail" , "column" , "One of a number of closely spaced supports for a railing" , "Pillar supporting staircase rail" , "Pillar supporting a rail, may be rustable" .)