Station alteration - good to be replaced with new line (7)
I believe the answer is:
channel
'station alteration good' is the definition.
'channel' can be an answer for 'station' (TV channel). I'm unsure of the 'alteration good' bit.
'replaced with new line' is the wordplay.
'replaced with' becomes 'chan' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'new' becomes 'n' (abbreviation as in New Testament NT).
'line' becomes 'el' (el is a kind of line).
'chan'+'n'+'el'='CHANNEL'
'to be' acts as a link.
Can you help me to learn more?
(Other definitions for channel that I've seen before include "BBC1, say" , "Gutter; medium" , "Narrow body of water linking two larger bodies" , "Ditch" , "Length of water; radio station" .)