Nothing more than an old lake (4)
I believe the answer is:
mere
This clue could be a double definition.
'nothing more than' is the first definition.
(I've seen this before)
'an old lake' is the second definition.
'mere' can be an answer for 'lake' (mere is a kind of lake). I'm not certain of the 'an old' bit.
Can you help me to learn more?
(Other definitions for mere that I've seen before include "Nothing more than - water" , "Just an English lake" , "Nothing more than what is specified (4)" , "sea" , "Lake, pond (poetic)" .)