Have no time for rehearsal? Sound unhappy and move deliberately, perhaps (5-4)

I believe the answer is:

sight-read

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'have no time for rehearsal?' is the definition.
I can't tell whether this defines the answer.

'sound unhappy and move deliberately perhaps' is the wordplay.
I cannot really understand how this works, but
'sound' could be 'sigh' (sigh is a kind of sound) and 'sigh' is found within the answer.
'move' could be 'tread' (tread is a kind of move) and 'tread' is found within the answer.
No letters remain.
This explanation may well be incorrect...

Can you help me to learn more?

(Another definition for sight-read that I've seen is " Interpret music without preparation".)

I've seen this clue in The Independent.

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