He has nothing to add to the list (5)
I believe the answer is:
rollo
'he has nothing' is the definition.
The definition suggests a singular noun which matches the answer.
'add to the list' is the wordplay.
'add' becomes 'o' (I can't explain this - if you can you should believe this answer much more).
'to' says to put letters next to each other (I've seen this in other clues).
'the list' becomes 'roll' (roll is a kind of list).
'o' put after 'roll' is 'ROLLO'.
'to' acts as a link.
Can you help me to learn more?
(Other definitions for rollo that I've seen before include "First duke of Normandy" , "Viking founder of Normandy dukedom" , "Boy" .)