Straight line, we hear, in Cornish city (5)
I believe the answer is:
truro
'cornish city' is the definition.
(I've seen this before)
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for truro that I've seen before include "English city" , "Cornish market town" , "City of Cornwall" , "Cornish county town" , "One slain by Cain" .)