Possibly Alf has clay to mould, but you may not believe it (7)
I believe the answer is:
fallacy
'mould but you may not believe it' is the definition.
I can't judge whether this definition defines the answer.
'possibly alf has clay' is the wordplay.
'possibly' indicates an anagram.
'has' says to put letters next to each other.
'alf' after 'clay' is 'clayalf'.
'clayalf' anagrammed gives 'FALLACY'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for fallacy that I've seen before include "False or mistaken belief." , "Wrong but prevalent notion" , "A mistake in reasoning" , "Faulty reasoning or mistaken belief" , "The old lie" .)