Such a solution could upset aliens (6)
I believe the answer is:
saline
'such a solution' is the definition.
(chemical solution)
'upset aliens' is the wordplay.
'upset' is an anagram indicator.
'aliens' anagrammed gives 'SALINE'.
'could' is the link.
(Other definitions for saline that I've seen before include "Salty like sea water" , "Containing or tasting of salt" , "Like contents of cellar" , "Containing salt for aliens" , "Salty to aliens" .)