Most gullible Scot brought back waistcoat from US (7)
I believe the answer is:
naivest
'most gullible' is the definition.
('naive' can be similar in meaning to 'gullible')
'scot brought back waistcoat from us' is the wordplay.
'scot' becomes 'ian' (Scottish given name).
'brought back' says the letters should be written in reverse.
'waistcoat from us' becomes 'vest' (US term for a waistcoat).
'ian' reversed gives 'nai'.
'nai'+'vest'='NAIVEST'
(Other definitions for naivest that I've seen before include "far from the most wise?" , "Most unsophisticated" , "Most simple" , "very green" , "Least sophisticated" .)