Separate if seen in street (4)
I believe the answer is:
sift
'separate' is the definition.
(sifting is a kind of separating)
'if seen in street' is the wordplay.
'seen in' means one lot of letters goes inside another.
'street' becomes 'st'.
'if' put inside 'st' is 'SIFT'.
(Other definitions for sift that I've seen before include "Examine minutely" , "Separate out - examine closely" , "Use a sieve" , "Examine (eg evidence) discriminatingly" , "Remove lumps from, eg, flour" .)