The sort of alibi which has not been forged (4,4)
I believe the answer is:
cast iron
'alibi which has not been forged' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how they can define each other.
'the sort of' is the wordplay.
I cannot quite see how this works, but
'the' could be 't' (the is pronounced as a 't' sound in some dialects) and 't' is found within the answer.
'of' could be 'o' and 'o' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for cast iron that I've seen before include "Solid, robust" , "Strong" , "Unyielding" , "such an alibi?" , "Firm and unchangeable" .)