Firm line taken for depression (3)
I believe the answer is:
col
'taken for depression' is the definition.
'col' can be an answer for 'depression' (geographical term for a low point). I'm not sure about the 'taken for' bit.
'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'
Can you help me to learn more?
(Other definitions for col that I've seen before include "Somewhere up on the mountain" , "Low area between peaks" , "route through mountains" , "Feature of mountain" , "Low point of a ridge" .)