American — one with capacity to be good-natured (7)
I believe the answer is:
amiable
'be good-natured' is the definition.
('amiable' can be similar in meaning to 'good-natured')
'american one with capacity' is the wordplay.
'american' becomes 'am' (abbreviation in some contexts).
'one' becomes 'i' (Roman numeral).
'with capacity' becomes 'able'.
'am'+'i'+'able'='AMIABLE'
'to' acts as a link.
(Other definitions for amiable that I've seen before include "Friendly, kindly" , "'Affable, genial (7)'" , "Friendly and good-natured (7)" , "Charming" , "Good-humoured" .)