Least sophisticated doctor in a singlet (7)
I believe the answer is:
naivest
'least sophisticated' is the definition.
('naive' is the opposite of 'sophisticated' so 'naivest' should be a synonym for 'least sophisticated')
'doctor in a singlet' is the wordplay.
'doctor' is an anagram indicator.
'singlet' becomes 'vest' (synonyms).
'in'+'a'='ina'
'ina' is an anagram of 'nai'.
'nai'+'vest'='NAIVEST'
(Other definitions for naivest that I've seen before include "very green" , "Most open" , "Least sophisticated" , "Most simple" , "Most unsophisticated" .)