Because it's in place of (3)
I believe the answer is:
for
I believe this clue is a double definition.
'because' is the first definition.
(I've seen this before)
'in place of' is the second definition.
(I've seen this in another clue)
'it's' is the link.
(Other definitions for for that I've seen before include "In favour of; since" , "On behalf of" , "On the side of" , "In support of" , "Pro" .)