Jill's partner gets alien garment (6)
I believe the answer is:
jacket
'garment' is the definition.
(I know that jacket is a more specific form of the action garment)
'jill's partner gets alien' is the wordplay.
'jill's partner' becomes 'jack' (I can't justify this - if you can you should give a lot more credence to this answer).
'gets' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extraterrestrial).
'jack'+'et'='JACKET'
Can you help me to learn more?
(Other definitions for jacket that I've seen before include "Record sleeve" , "Short coat or outer skin" , "Outer skin of a potato" , "See 1" , "Outer garment or casing" .)