Aliens work out solution (6)
I believe the answer is:
saline
'solution' is the definition.
(saline is a kind of solution)
'aliens work out' is the wordplay.
'work out' indicates an anagram.
'aliens' anagrammed gives 'SALINE'.
(Other definitions for saline that I've seen before include "like the Mediterranean?" , "Salty like sea water" , "Intravenous feeding" , "Like contents of cellar" , "Containing or tasting of salt" .)