Prison guard's twitch? (9)

I believe the answer is:

birdwatch

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'twitch?' is the definition.
Although both the answer and definition are verbs in their base form, I cannot understand how one could define the other.

'prison guard's' is the wordplay.
'prison' becomes 'bird'.
'guard' becomes 'watch' (watch is a kind of guard).
'bird'+'watch'='BIRDWATCH'

Can you help me to learn more?

I've seen this clue in The Independent.

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