Outlaw is reverse of popular, gatecrashing party (6)
I believe the answer is:
banish
'outlaw' is the definition.
(I've seen this before)
'reverse of popular gatecrashing party' is the wordplay.
'reverse of' says the letters should be written backwards.
'popular' becomes 'in'.
'gatecrashing' indicates putting letters inside.
'party' becomes 'bash' (bash is a kind of party).
'in' back-to-front is 'ni'.
'ni' going inside 'bash' is 'BANISH'.
'is' acts as a link.
(Other definitions for banish that I've seen before include "Exile (someone)" , "Dismiss, send away" , "Drive away, maybe from country" , "Deport, exile" , "Condemn to exile" .)