Connecting line (4)
I believe the answer is:
seam
'connecting line' is the definition.
(I've seen this in another clue)
This is all the clue.
(Other definitions for seam that I've seen before include "Underground layer of mineral" , "Stratum of ore or coal" , "Join round cricket ball" , "Unfasten" , "Line of sewing - stratum of coal" .)