Small pool of rainwater (6)
I believe the answer is:
puddle
'small pool of rainwater' is the definition.
(I've seen this before)
This is the entire clue.
(Other definitions for puddle that I've seen before include "Little bit of water" , "Small body of standing water" , "Small amount of standing rainwater" , "Small pool of rainwater" , "Small pool of muddy water" .)