Performer outside in Alien following extremely careful player (12)
I believe the answer is:
clarinettist
This clue could be a double definition.
'performer' is the first definition.
(clarinettist is a kind of performer)
'outside in alien following extremely careful player' is the second definition.
'clarinettist' can be an answer for 'player' (clarinettist is a kind of player). I am not sure about the 'outside in alien following extremely careful' bit.
Can you help me to learn more?
(Other definitions for clarinettist that I've seen before include "Cats in litter (anag) -- woodwind player" , "Musician, like Benny Goodman, say" .)