Offer alien a bathroom facility (5)
I believe the answer is:
bidet
'a bathroom facility' is the definition.
(bidet is a bathroom appliance)
'offer alien' is the wordplay.
'offer' becomes 'bid' (synonyms).
'alien' becomes 'et' (extra terrestrial).
'bid'+'et'='BIDET'
(Other definitions for bidet that I've seen before include "Washing facility" , "bathroom cleaner" , "Bathroom feature" , "Bathroom facility" , "Bathroom item" .)