Army taking on a tricky opening mission (8)
I believe the answer is:
legation
'mission' is the definition.
(legation is a kind of mission)
'army taking on a tricky opening' is the wordplay.
'army' becomes 'legion' (legion is a kind of army**).
'taking on' indicates putting letters inside.
'opening' says to take the initial letters.
The initial letter of 'tricky' is 't'.
'a'+'t'='at'
'legion' enclosing 'at' is 'LEGATION'.
(Other definitions for legation that I've seen before include "Official residence of diplomat and staff" , "Diplomatic group or mission" , "Embassy" , "A long tie (anag.)" , "Diplomatic minister and staff, and their residence" .)