Rapid growth ruining a coastline (10)
I believe the answer is:
escalation
'rapid growth' is the definition.
(I've seen this before)
'ruining a coastline' is the wordplay.
'ruining' is an anagram indicator.
'a'+'coastline'='acoastline'
'acoastline' is an anagram of 'ESCALATION'.
(Other definitions for escalation that I've seen before include "Rapid rise in level" , "sudden rise" , "Rapid growth" , "expansion" .)