Civilian militia, unfit for training, it's initially ... (5)
I believe the answer is:
mufti
'civilian' is the definition.
(I've seen this before)
'militia unfit for training it's initially' is the wordplay.
'initially' suggests taking the first letters.
The first letters of 'militia unfit for training its' is 'MUFTI'.
(Other definitions for mufti that I've seen before include "Civvy dress" , "Non-military clothing" , "No official dress" , "For soldiers, civilian dress" , "Plain clothes - Muslim law expert" .)