Certain to miss first body of water (3)
I believe the answer is:
ure
'body of water' is the definition.
'certain to miss first' is the wordplay.
'certain' becomes 'sure' (synonyms).
'to miss first' means to remove the first letter.
'sure' with its initial letter taken away is 'URE'.
(Other definitions for ure that I've seen before include "North Yorkshire river" , "Midge --, singer" , "River of N. Yorkshire" , "Wensleydale river" , "Ripon's river" .)