Rapid growth ruined a coastline (10)
I believe the answer is:
escalation
'rapid growth' is the definition.
(I've seen this before)
'ruined a coastline' is the wordplay.
'ruined' indicates anagramming the letters.
'a'+'coastline'='acoastline'
'acoastline' with letters rearranged gives 'ESCALATION'.
(Other definitions for escalation that I've seen before include "expansion" , "Rapid rise in level" , "Rapid growth" , "sudden rise" .)