Picnicker's worry (4)
I believe the answer is:
rain
'picnicker's worry' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for rain that I've seen before include "Wet weather" , "See 12" , "The ---, it ---eth every day (T.Night)" , "previous rule?" , "Pelt" .)