Setter regarding lake (4)
I believe the answer is:
mere
'lake' is the definition.
(mere is a kind of lake)
'setter regarding' is the wordplay.
'setter' becomes 'me' (the setter of the clue).
'regarding' becomes 're'.
'me'+'re'='MERE'
(Other definitions for mere that I've seen before include "Lake - French mother" , "Nothing more than a small pond" , "Lake (poetic); Maori war club" , "Simply, nothing more than" , "Lake in Cumbria" .)