Adder, and when to see it? (6)
I believe the answer is:
summer
'adder' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
(Other definitions for summer that I've seen before include "` `Now is the winter of our discontent / Made glorious ... by this sum of York' ' (Richard III)" , "'Now is the winter of our discontent / Made glorious ... by this sun of York' (Richard III)" , "Period of heat" , "Late singer" , "Would one count" .)