What's seen during summers: no, winters! (4)
I believe the answer is:
snow
'what's seen' is the definition.
I can't tell whether this definition defines the answer.
'during summers no winters' is the wordplay.
'during' says the answer is hidden in the clue.
'SNOW' is hidden within 'summers no winters'.
(Other definitions for snow that I've seen before include "Cold precipitation" , "Basic White Christmas ingredient" , "Weather in flakes" , "feature of 27 [THE BIG FREEZE]" , "Frozen rain" .)