Only some returned: it's a trick (4)

I believe the answer is:

trap

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

'it's a trick' is the definition.
(I've seen this in another clue)

I cannot understand how the remainder of the clue works.

Can you help me to learn more?

(Other definitions for trap that I've seen before include "Part of a snare" , "Rock; carriage; gin" , "Snaring device" , "Snare; pony carriage" , "Control ball" .)

I've seen this clue in The Irish Independent.

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