Crossword Genius

Quiet period back on terra firma depresses child (3-6)

Ross

I believe the answer is:

off-season

I'm a little stuck... Click here to teach me more about this clue!

'quiet period' is the definition.
'off season' can be an answer for 'period' (I have seen 'slack period' mean 'off season' so perhaps 'period' could also mean 'off season'). I am unsure of the 'quiet' bit.

'back on terra firma depresses child' is the wordplay.
I cannot really see how this works, but
'on' is within the answer.
This explanation may well be incorrect...

Can you help me to learn more?

I've seen this clue in The Independent.
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