Lake seen in summer excursion (4)
I believe the answer is:
mere
'lake' is the definition.
(I know that mere is a type of pond)
'in summer excursion' is the wordplay.
'in' says the answer is hidden in the clue.
'MERE' is hidden within 'summer excursion'.
'seen' is the link.
(Other definitions for mere that I've seen before include "Lake, pond (poetic)" , "Pure; lake" , "President Hollande's mother?" , "DDwater" , "Nothing more than a small pond" .)