Unfamiliar street to an extent (7)
I believe the answer is:
strange
'unfamiliar' is the definition.
('strange' can be similar in meaning to 'unfamiliar')
'street to an extent' is the wordplay.
'street' becomes 'st'.
'to an extent' becomes 'range' (range is a kind of extent).
'st'+'range'='STRANGE'
(Other definitions for strange that I've seen before include "Unknown or peculiar" , "Surreal" , "Odd kind of garnets" , "Bizarre" , "Different" .)