Noise or otherwise, it's a feature of coastlines (7)
I believe the answer is:
erosion
'a feature of coastlines' is the definition.
(coastlines erode)
'noise or otherwise' is the wordplay.
'otherwise' is an anagram indicator.
'noise'+'or'='noiseor'
'noiseor' is an anagram of 'EROSION'.
'it's' acts as a link.
(Other definitions for erosion that I've seen before include "The wearing away by water or weather" , "Disintegration" , "Process of wearing away, by water or time" , "The wearing away by weather 97)" , "Gradual decline" .)