It's less than his whole name (3)
I believe the answer is:
len
'it's less than his' is the definition.
I know nothing about this answer so I cannot tell whether this works.
'whole name' is the wordplay.
'whole' becomes 'le' (I can't justify this - if you can you should believe this answer much more).
'name' becomes 'n' (common abbreviation e.g. on forms).
'le'+'n'='LEN''
Can you help me to learn more?
(Other definitions for len that I've seen before include "- - Hutton, cricketer" , "Sir - - Hutton" , "Boy" , "Leonard (dim.)" , "-- Deighton, author" .)