Line brought into use when one rings (4,4)
I believe the answer is:
bell rope
'one rings' is the definition.
I know nothing about this answer so I can't tell whether it can be defined by this definition.
'line brought into use' is the wordplay.
'line' becomes 'bell' (bell is a kind of line).
'brought into use' becomes 'rope' (I can't justify this - if you can you should give a lot more credence to this answer).
'bell'+'rope'='BELL-ROPE'
'when' is the link.
Can you help me to learn more?
(Other definitions for bell rope that I've seen before include "line to carry out changes?" , "It hangs in the tower" .)