Connection used by school in Karachi (4)
I believe the answer is:
link
'connection' is the definition.
(link is a kind of connection)
'used by school in karachi' is the wordplay.
'used by' indicates a hidden word.
'LINK' is hidden within 'school in karachi'.
(Other definitions for link that I've seen before include "Connecting part" , "One ring or loop of a chain" , "Chain component" , "Relationship; intertwine" , "Bond" .)