Crossword Genius

Mull up! One left to take and zero back pain

Ross

I believe the answer is:

lumbago

I'm a little stuck... Click here to teach me more about this clue!

'back pain' is the definition.
(I've seen this in another clue)

'mull up one left to take and zero' is the wordplay.
I cannot really understand how this works, but
'one' could be 'a' and 'a' is found within the answer.
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
'zero' could be 'o' (O resembles 0) and 'o' is present in the answer.
This explanation may well be incorrect...

Can you help me to learn more?

(Other definitions for lumbago that I've seen before include "Painful condition" , "Pain -- a bum log (anag)" , "Lower back ache" , "Gaul mob (anag.)" , "Back problem" .)

I've seen this clue in the Sydney Morning Herald.
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