Heather, full of terrible dread, is running (9)

I believe the answer is:

laddering

'is running' is the definition.
(I've seen this before)

'heather full of terrible dread' is the wordplay.
'heather' becomes 'ling' ('ling' can be a synonym of 'heather').
'full of' means one lot of letters goes inside another.
'terrible' indicates an anagram.
'dread' with letters rearranged gives 'adder'.
'ling' going around 'adder' is 'LADDERING'.

(Another definition for laddering that I've seen is " Running".)

I've seen this clue in The Telegraph.

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