Most unsophisticated Scotsman returned undergarment (7)
I believe the answer is:
naivest
'most unsophisticated' is the definition.
('naive' can be a synonym of 'unsophisticated')
'scotsman returned undergarment' is the wordplay.
'scotsman' becomes 'ian'.
'returned' shows that the letters should be reversed in order.
'undergarment' becomes 'vest' (vest is a kind of undergarment).
'ian' in reverse letter order is 'nai'.
'nai'+'vest'='NAIVEST'
(Other definitions for naivest that I've seen before include "very green" , "Most unsophisticated" , "Least sophisticated" , "far from the most wise?" , "Most open" .)