Said seven days demand was lacking in spirit (4-5)

I believe the answer is:

weak-kneed

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'was lacking in spirit' is the definition.
I can't judge whether this defines the answer.

'said seven days demand' is the wordplay.
'said' shows a homophone (sound like).
'seven days' becomes 'week' (I've seen this in another clue).
'demand' becomes 'kneed' (I can't justify this - if you can you should give a lot more credence to this answer).
'week' sounds like 'weak'.
'weak'+'kneed'='WEAK-KNEED'

Can you help me to learn more?

(Other definitions for weak-kneed that I've seen before include "Feeble" , "Lacking moral courage in legs?" .)

I've seen this clue in the Sydney Morning Herald.

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