Jill's partner gets alien garment (6)
I believe the answer is:
jacket
'garment' is the definition.
(jacket is a kind of garment)
'jill's partner gets alien' is the wordplay.
'jill's partner' becomes 'Jack' (as in Jack and Jill, nursery rhyme characters).
'gets' is a charade indicator (letters next to each other).
'alien' becomes 'et' (extraterrestrial).
'jack'+'et'='JACKET'
(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" .)