Possibly long calculation for army unit (8)
I believe the answer is:
division
I believe this clue is a double definition.
'possibly long calculation' is the first definition.
(as in long division)
'army unit' is the second definition.
(division is a kind of army unit)
'for' acts as a link.
(Other definitions for division that I've seen before include "Part of a whole, a section" , "Large army unit" , "Discord, split" , "part of force" , "A number of teams grouped together in sport according to ability" .)