Lamb, say, rambling is unfathomable (7)
I believe the answer is:
abysmal
'unfathomable' is the definition.
(thesaurus)
'lamb say rambling' is the wordplay.
'rambling' indicates an anagram.
'lamb'+'say'='lambsay'
'lambsay' with letters rearranged gives 'ABYSMAL'.
'is' acts as a link.
(Other definitions for abysmal that I've seen before include "Deplorable, extremely bad - say ignorance" , "Immeasurably bad" , "Very bad or bottomless" , "Really terrible" , "Dreadful" .)