A cat is adrift (6)
I believe the answer is:
astray
'adrift' is the definition.
(thesaurus)
'a cat' is the wordplay.
'cat' becomes 'stray' (stray is a kind of cat).
'a'+'stray'='ASTRAY'
'is' is the link.
(Other definitions for astray that I've seen before include "Wandering off the path" , "(Gone) in wrong direction" , "Away from the right path like a satyr (6)" , "Like carrier" , "In a lost state" .)