Check every letter in answer — or one in box, in case (8)
I believe the answer is:
testator
'one in box in case' is the definition.
The definition and answer can be both people as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'check every letter in answer or' is the wordplay.
I cannot quite understand how this works, but
'check' could be 'test' (as in testing or checking something works) and 'test' is found in the answer.
'letter' could be 't' and 't' is located in the answer.
'answer' could be 'a' (abbreviation eg in Q and A) and 'a' is located in the answer.
'or' is within the answer.
This accounts for all the letters.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for testator that I've seen before include "Person who makes a will" , "One willing" , "Attestor (anag.)" , "He's willing" , "Will provider" .)