Not only straight; into unclothed doll (6)
I believe the answer is:
barbie
'doll' is the definition.
(I have seen 'Popular doll' mean 'barbie' so perhaps 'doll' could also mean 'barbie')
'not only straight into unclothed' is the wordplay.
'not only straight' becomes 'bi' (I am not sure about this - if you are sure you should believe this answer much more).
'into' means one lot of letters goes inside another.
'unclothed' becomes 'bare' (similar in meaning).
'bi' inserted inside 'bare' is 'BARBIE'.
Can you help me to learn more?
(Other definitions for barbie that I've seen before include "Popular doll" , "Figure girl might dress up" , "Australian BBQ" , "meal out" , "Darwin's feeding party?" .)