Firm line shows way through mountains (3)
I believe the answer is:
col
'way through mountains' is the definition.
I can't tell whether this defines the answer.
'firm line' is the wordplay.
'firm' becomes 'co' (a firm is a company).
'line' becomes 'l' (used when specifying particular lines from a poem).
'co'+'l'='COL'
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for col that I've seen before include "Pass between hills" , "Mountain saddle" , "A pass between mountain peaks" , "Low area between peaks" , "Somewhere up on the mountain" .)