Fast broadband connection mostly in street (5)
I believe the answer is:
swift
'fast' is the definition.
(similar in meaning)
'broadband connection mostly in street' is the wordplay.
'broadband connection' becomes 'wi-fi' (wireless internet connection).
'mostly' means to remove the last letter.
'in' means one lot of letters goes inside another.
'street' becomes 'st' (abbreviation).
'wifi' with its final letter taken off is 'wif'.
'wif' inserted within 'st' is 'SWIFT'.
(Other definitions for swift that I've seen before include "Fleet" , "Fast; bird; writer" , "A bird; Gulliver author" , "Swallow-like bird" , "Happening quickly; author" .)