The season for the adder (6)
I believe the answer is:
summer
I believe this clue is a double definition.
'the season' is the first definition.
(summer is a season)
'the adder' is the second definition.
(I've seen this before)
'for' acts as a link.
(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)" , "'Shall I compare thee to a . . . . . .'s day?' (Shakespeare)" , "could be described as Indian" , "Best of times" , "Adder" .)