We start building a trap (3)

I believe the answer is:

web

'a trap' is the definition.
(web is a kind of trap)

'we start building' is the wordplay.
'start' suggests taking the first letters.
The first letter of 'building' is 'b'.
'we'+'b'='WEB'

(Other definitions for web that I've seen before include "Spider's trap" , "Complicated plot" , "Internet site for spider?" , "Network of fine threads" , "Network, may be World Wide" .)

I've seen this clue in the Evening Standard.

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