Crossword Genius

Lamb, say, rambling is unfathomable (7)

Ross

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" .)

I've seen this clue in The Telegraph.
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