Incomplete arrest following one northern copper's nightmare (7)
I believe the answer is:
incubus
'nightmare' is the definition.
(synonyms)
'incomplete arrest following one northern copper's' is the wordplay.
'incomplete' means to remove the last letter.
'arrest' becomes 'bust'.
'following' says to put letters next to each other.
'one northern' becomes 'in' (I am not sure about this - if you are sure you should believe this answer much more).
'copper' becomes 'Cu' (chemical symbol).
'bust' with its last letter taken away is 'bus'.
'in'+'cu'='incu'
'bus' put after 'incu' is 'INCUBUS'.
(Other definitions for incubus that I've seen before include "Oppressive burden" , "Nightmare - cub in us (anag)" , "Oppressive nightmare" , "Evil spirit visiting a sleeping person" , "Burdensome person or evil spirit visiting sleeper" .)