Insignificant lake (4)
I believe the answer is:
mere
This clue is a double definition.
'insignificant' is the first definition.
(mere can mean unimportant or trivial)
'lake' is the second definition.
(I know that mere is a type of pond)
(Other definitions for mere that I've seen before include "Nothing more than what is specified (4)" , "Simply, nothing more than" , "Lake, pond (poetic)" , "Lake in Cumbria" , "DDwater" .)