Keen army officer accepting the last word (6)
I believe the answer is:
lament
'keen' is the definition.
('lament' can be a synonym of 'keen')
'army officer accepting the last word' is the wordplay.
'army officer' becomes 'lt' (I've seen this in other clues).
'accepting' means one lot of letters goes inside another.
'the last word' becomes 'amen' (I've seen this before).
'lt' placed around 'amen' is 'LAMENT'.
(Other definitions for lament that I've seen before include "Sad song" , "Mourn in mental way (6)" , "Wail" , "Mourn, grieve for" , "Cry of sorrow" .)