Stop going the wrong way in crowd - it's lunacy (7)
I believe the answer is:
madness
'lunacy' is the definition.
('madness' can be a synonym of 'lunacy')
'stop going the wrong way in crowd' is the wordplay.
'stop going' becomes 'end' (I can't justify this - if you can you should give a lot more credence to this answer).
'the wrong way' is a reversal indicator.
'in' indicates putting letters inside.
'crowd' becomes 'mass' (massing is a kind of crowding).
'end' back-to-front is 'dne'.
'dne' going within 'mass' is 'MADNESS'.
'it's' is the link.
Can you help me to learn more?
(Other definitions for madness that I've seen before include "Utter foolishness" , "Lunacy - ska/pop band" , "Group" , "Insanity" , "Absurdity" .)