Ian's retro sleeveless top is most unsophisticated! (7)
I believe the answer is:
naivest
'most unsophisticated' is the definition.
('naive' can be similar in meaning to 'unsophisticated')
'ian's retro sleeveless top' is the wordplay.
'retro' says the letters should be written in reverse (retro is Latin for 'backwards').
'sleeveless top' becomes 'vest' (I've seen this in another clue).
'ian' reversed gives 'nai'.
'nai'+'vest'='NAIVEST'
'is' acts as a link.
(Other definitions for naivest that I've seen before include "Most unsophisticated" , "far from the most wise?" , "Least sophisticated" , "very green" , "Most simple" .)