A firm way with a strong line (4)
I believe the answer is:
cord
'strong line' is the definition.
'cord' can be an answer for 'line' (cord is a kind of line). I'm not certain of the 'strong' bit.
'a firm way' is the wordplay.
'a firm' becomes 'co' (a firm is a company).
'way' becomes 'rd' (short for road).
'co'+'rd'='CORD'
'with a' acts as a link.
Can you help me to learn more?
(Other definitions for cord that I've seen before include "Line of twisted fibres or threads" , "Thin string" , "Small rope or thick string" , "A measure of cut wood" , "Small rope; flex" .)