John, name for diver (4)
I believe the answer is:
loon
'diver' is the definition.
(I know that diver can be written as loon)
'john name' is the wordplay.
'john' becomes 'loo' (loo is a kind of john).
'name' becomes 'n' (common abbreviation e.g. on forms).
'loo'+'n'='LOON'
'for' acts as a link.
(Other definitions for loon that I've seen before include "Simple-minded person" , "Silly person (colloq.)" , "Fish-eating diving bird" , "Diving bird with wild cry - sounds crazy" , "US bird is also a silly person?" .)