Corruptly closing river in the season (8)

I believe the answer is:

vernally

'in the season' is the definition.
(vernal means relating to spring)

'corruptly closing river' is the wordplay.
'corruptly' becomes 'venally'.
'closing' is an insertion indicator ('closing' can also mean 'enclosing').
'river' becomes 'r'.
'venally' enclosing 'r' is 'VERNALLY'.

I've seen this clue in The Times.

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