Bad weather in August or May (5)
I believe the answer is:
storm
'bad weather' is the definition.
(I've seen this before)
'in august or may' is the wordplay.
'in' indicates a hidden word.
'STORM' is hidden in the letters of 'august or may'.
(Other definitions for storm that I've seen before include "violent assault" , "rave" , "X-Men character" , "Assault (with troops)" , "Shout angrily, assault" .)