Dull summer, say, when there's no jumping (4,6)

I believe the answer is:

flat season

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'there's no jumping' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.

'dull summer say' is the wordplay.
'dull' becomes 'flat' (synonyms).
'summer say' becomes 'season' (summer is a kind of season).
'flat'+'season'='FLAT SEASON'

'when' is the link.

Can you help me to learn more?

(Another definition for flat season that I've seen is " no time for jumpers?".)

I've seen this clue in The Daily Express.

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