Northern university to sort out type of weapon (7)
I believe the answer is:
nuclear
'type of weapon' is the definition.
'northern university to sort out' is the wordplay.
'northern' becomes 'n' (abbreviation e.g. N. Ireland).
'university' becomes 'U' (abbreviation as in UMass).
'to sort out' becomes 'clear' ('clear' can be a synonym of 'sort out').
'n'+'u'+'clear'='NUCLEAR'
(Other definitions for nuclear that I've seen before include "Such power available" , "small family?" , "Concerning subatomic particles" , "Type of energy" , "These weapons release atomic energy" .)