A quartet, cosily placed, is least affected (7)
I believe the answer is:
naivest
'least affected' is the definition.
(naive can mean unaffected or innocent)
'a quartet cosily placed' is the wordplay.
I cannot really see how this works, but
'a' is present in the answer.
'quartet' could be 'iv' (Roman numerals for four) and 'iv' is found within the answer.
The remaining letters 'nest' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'is' acts as a link.
(Other definitions for naivest that I've seen before include "far from the most wise?" , "very green" , "Most open" , "Most simple" , "Least sophisticated" .)