London band finish up in service (7)
I believe the answer is:
madness
'service' is the definition.
Although both the answer and definition are singular nouns, I don't see how they can define each other.
'london band finish up in' is the wordplay.
'london band' becomes 'mass' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'finish' becomes 'end' ('end' can be a synonym of 'finish').
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'in' is an insertion indicator.
'end' written backwards gives 'dne'.
'mass' enclosing 'dne' is 'MADNESS'.
Can you help me to learn more?
(Other definitions for madness that I've seen before include "Absurdity" , "Dementia" , "fury" , "Group" , "Lunacy - ska/pop band" .)