He should put the right letters in his case (10)
I believe the answer is:
typesetter
'he should put the right' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'letters in his case' is the wordplay.
'letters' becomes 'type' (I've seen this in another clue).
'in his case' becomes 'setter' (I can't justify this - if you can you should give a lot more credence to this answer).
'type'+'setter'='TYPESETTER'
Can you help me to learn more?
(Other definitions for typesetter that I've seen before include "whose work will leave a lasting impression?" , "Press worker" , "Printing expert" , "print worker" , "One doing composition" .)