Mull up! One left to take and zero back pain
I believe the answer is:
lumbago
'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" .)