Mass behind small hill in bad weather (5)
I believe the answer is:
storm
'bad weather' is the definition.
(I've seen this before)
'mass behind small hill in' is the wordplay.
'mass' becomes 'm' (symbol used in Physics e.g. f=ma).
'behind' is a charade indicator (letters next to each other).
'small' becomes 's' (abbreviation - e.g. clothes size).
'hill' becomes 'tor' (tor is a kind of hill).
'in' is an insertion indicator.
'm' after 's' is 'sm'.
'sm' enclosing 'tor' is 'STORM'.
(Other definitions for storm that I've seen before include "Rumpus" , "Violent weather condition" , "violent assault" , "Wind force 10 on Beaufort scale" , "Feature of weather" .)