Small lake
There are a few possibilities for this:
'small lake' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pool that I've seen before include "American game resembling snooker" , "Game played on a table with six pockets" , "Put (resources) together" , "share" , "Small area of still water" .)
'small lake' is the definition.
(term for a mountain lake)
This is the entire clue.
(Other definitions for tarn that I've seen before include "Mountain pool: French department" , "Small mountain lake" , "High water" , "Scottish mountain lake" .)
'small lake' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for pond that I've seen before include "Small body of water" , "Small lake" , "Himalayan country" , "Pool of still water" , "Small area of still water" .)