Spa output that's seasonal (11)

I believe the answer is:

springwater

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'seasonal' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.

'spa output' is the wordplay.
'spa' becomes 'spring' (I've seen this before).
'output' becomes 'water' (I am not sure about this - if you are sure you should believe this answer much more).
'spring'+'water'='SPRINGWATER'

'that's' acts as a link.

Can you help me to learn more?

I've seen this clue in The Mirror.

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