Backache? Penny has to leave plant (7)
I believe the answer is:
lumbago
'backache?' is the definition.
(lumbago is a kind of backache)
'penny has to leave plant' is the wordplay.
I cannot really understand how this works, but
'leave' could be 'go' and 'go' is found 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 "Gaul mob (anag.)" , "Maul bog badly for a rheumatic affliction" , "Rheumatic backache" , "Rheumatism of lower back" , "Painful condition" .)