Lean period when not working (5,4)

I believe the answer is:

spare time

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'not working' is the definition.
Although both the answer and definition are singular nouns, I cannot see how they can define each other.

'lean period' is the wordplay.
'lean' becomes 'spare' (similar in meaning).
'period' becomes 'time' (period of time).
'spare'+'time'='SPARE TIME'

'when' acts as a link.

Can you help me to learn more?

(Other definitions for spare time that I've seen before include "Prime seat for leisure period" , "Leisure moments" , "When we can please ourselves what we do" , "Leisure periods" , "Free period, no commitments (5,4)" .)

I've seen this clue in The Globe and Mail.

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