It left damage in extremely treacherous hot coastal wetland (4,5)
I believe the answer is:
salt marsh
'wetland' is the definition.
(I know that salt marsh is a type of fen)
'it left damage in extremely treacherous hot coastal' is the wordplay.
I cannot really understand how this works, but
'it' could be 'sa' (old-fashioned abbreviation for sex appeal) and 'sa' is located in the answer.
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'damage' could be 'mars' (I've seen this in another clue) and 'mars' is present in the answer.
'hot' could be 'h' (abbreviation eg on taps) and 'h' is located in the answer.
A single letter 't' remains which might be clued in a way I don't understand.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for salt marsh that I've seen before include "area near the sea" , "Land sodden with sea water" , "regularly flooded area" , "grazing area" , "Boggy ground" .)