Calamity if climbing rocky coast shaking tons (6)
I believe the answer is:
fiasco
'calamity' is the definition.
(I've seen this before)
'if climbing rocky coast shaking tons' is the wordplay.
'climbing' is a reversal indicator (in a down clue, letters climb up).
'rocky' is an anagram indicator.
'shaking' indicates named letters should be taken away (to shake can mean to lose or get rid of).
'tons' becomes 't'.
'if' in reverse letter order is 'fi'.
'coast' with 't' removed is 'coas'.
'coas' anagrammed gives 'asco'.
'fi'+'asco'='FIASCO'
(Other definitions for fiasco that I've seen before include "washout" , "Embarrassing failure" , "Sudden and complete collapse" , "A ludicrous failure, a shambles" , "Utter failure, debacle" .)