Unreasonable aliens spun a record (5)
I believe the answer is:
steep
'unreasonable' is the definition.
(thesaurus)
'aliens spun a record' is the wordplay.
'aliens' becomes 'ETs' (ET is short for extraterrestrial).
'spun' says the letters should be written in reverse.
'a record' becomes 'EP' (abbreviation for extended play record).
'ets' back-to-front is 'ste'.
'ste'+'ep'='STEEP'
(Other definitions for steep that I've seen before include "Rapidly rising" , "Hard for cycling" , "Soak; expensive (price)" , "Unreasonably dear - soak" , "Marinate" .)