Body of water in summer enlarged (4)
I believe the answer is:
mere
'body of water' is the definition.
(mere is a kind of body of water)
'in summer enlarged' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MERE' can be found hidden inside 'summer enlarged'.
(Other definitions for mere that I've seen before include "Pond, only what is said" , "Lake; simple" , "Simply, nothing more than" , "Pool -- pure and simple" , "Unimpressive" .)