Yours truly on lake (4)
I believe the answer is:
mere
'lake' is the definition.
(mere is a kind of lake)
'yours truly on' is the wordplay.
'yours truly' becomes 'me'.
'on' becomes 're' ('on' can be mean 'with reference to'**).
'me'+'re'='MERE'
(Other definitions for mere that I've seen before include "of no significance" , "Bare, simple" , "Nothing more than what is specified (4)" , "Lake; Wiltshire town" , "Pond, only what is said" .)