Separate if in the way (4)
I believe the answer is:
sift
'separate' is the definition.
(sifting is a kind of separating)
'if in the way' is the wordplay.
'in the' means one lot of letters goes inside another.
'way' becomes 'st' (abbreviation for street).
'if' inserted inside 'st' is 'SIFT'.
(Other definitions for sift that I've seen before include "screen" , "Make fist of filter" , "Sort carefully" , "Use fist to pass through perforated container" , "Winnow out" .)