What summers can do to a stile that's rotting? (8)

I believe the answer is:

totalise

'what summers can do' is the definition.
(someone doing 'sums' might totalise numbers)

'to a stile that's rotting?' is the wordplay.
'that's rotting?' indicates an anagram.
'a'+'stile'='astile'
'astile' with letters rearranged gives 'talise'.
'to'+'talise'='TOTALISE'

I've seen this clue in The Guardian.

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