Ruin, perhaps as a march crushes Left (8)
I believe the answer is:
demolish
'ruin' is the definition.
(thesaurus)
'as a march crushes left' is the wordplay.
I cannot really understand how this works, but
'march' could be 'demo' (as in a demonstration) and 'demo' is found in the answer.
'left' could be 'l' (common abbreviation) and 'l' is found in the answer.
The remaining letters 'ish' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
'perhaps' acts as a link.
Can you help me to learn more?
(Other definitions for demolish that I've seen before include "Pull or tear down" , "Knock down, say a building" , "Break down" , "Knock down, a building or argument perhaps" , "Knock or tear down" .)