Diver in lake having nothing on (4)
I believe the answer is:
loon
'diver' is the definition.
('loon' can be a synonym of 'diver')
'lake having nothing on' is the wordplay.
'lake' becomes 'l'.
'having nothing' becomes 'o' (I've seen this in other clues).
'l'+'o'+'on'='LOON'
'in' acts as a link.
(Other definitions for loon that I've seen before include "Noddy" , "Diving bird with wild cry - sounds crazy" , "N American diving bird" , "US bird is also a silly person?" , "Fool" .)