Just visible in summer, especially (4)
I believe the answer is:
mere
'just' is the definition.
('just' can mean 'mere' or 'only')
'visible in summer especially' is the wordplay.
'visible in' indicates a hidden word.
'MERE' is hidden within 'summer especially'.
(Other definitions for mere that I've seen before include "Nothing more than a small pool" , "Lake, pond (poetic)" , "Paltry" , "See 13" , "Pond - pure and simple" .)