Stray in the summer rain (3)
I believe the answer is:
err
'stray' is the definition.
(to err is to stray or go wrong)
'in the summer rain' is the wordplay.
'in' indicates a hidden word.
'ERR' can be found hidden inside 'summer rain'.
(Other definitions for err that I've seen before include "'To . . . is human, to forgive divine'" , "Blunder" , "Deviate" , "do something you shouldn't" , "Miscalculate" .)