Personnel weapon exploded in mortar's casing (8)
I believe the answer is:
manpower
'personnel' is the definition.
(manpower is a kind of personnel)
'weapon exploded in mortar's casing' is the wordplay.
'exploded' is an anagram indicator.
'in' means one lot of letters goes inside another.
'casing' means to remove the middle letters (the outside letters are the case of the word).
'mortar' with its centre removed is 'mr'.
'weapon' anagrammed gives 'anpowe'.
'anpowe' put within 'mr' is 'MANPOWER'.
(Other definitions for manpower that I've seen before include "labour force" , "Work effort" , "Workforce" , "Staff resources" , "People available for work" .)