Following unknown into trap (4)

I believe the answer is:

'following' is the definition.
(I know that following can be written as next)

'unknown into trap' is the wordplay.
'unknown' becomes 'x' (algebra).
'into' means one lot of letters goes inside another.
'trap' becomes 'net' (net is a type of trap).
'x' inserted into 'net' is 'NEXT'.

(Other definitions for next that I've seen before include "shop" , "First in line" , "Coming nearest in order - come, please!" , "The one following or adjacent" , "Following in sequence" .)

I've seen this clue in The Guardian and The Telegraph.

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