We start building a trap (3)
I believe the answer is:
'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'.
(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" .)