Terrible weather for every small hill, in sum (10)

'terrible weather' is the definition.
(a superstorm is a very destructive storm)

'every small hill in sum' is the wordplay.
'every' becomes 'per' ('per' means 'for each' or 'for every').
'small' becomes 's' (abbreviation - e.g. clothes size).
'hill' becomes 'tor' (tor is a kind of hill).
'in' indicates putting letters inside.
'per'+'s'+'tor'='perstor'
'perstor' going within 'sum' is 'SUPERSTORM'.

'for' is the link.