On march noticing English have left is disheartening (12)

I believe the answer is:
demoralising
'disheartening' is the definition.
(synonyms)
'on march noticing english have left is' is the wordplay.
I cannot really understand how this works, but
'march' could be 'demo' (as in a demonstration) and 'demo' is present in the answer.
'left' could be 'l' (common abbreviation) and 'l' is found within the answer.
'is' is present in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for demoralising that I've seen before include "Causing loss of hope" , "Causing (someone) to lose hope" .)
