Lake seen in summer excursion (4)
I believe the answer is:
mere
'lake' is the definition.
(I know that mere is a type of lake)
'in summer excursion' is the wordplay.
'in' indicates the answer is hidden within the clue.
'MERE' is hidden in the letters of 'summer excursion'.
'seen' acts as a 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" .)