Register of leaderless soldiers with inadequate function (5)
I believe the answer is:
enrol
'register' is the definition.
(enrolling is a kind of registering)
'soldiers with inadequate function' is the wordplay.
'soldiers with' becomes 'en' (I can't explain this - if you can you should give a lot more credence to this answer).
'inadequate' means to remove the last letter (only some of the word appears).
'function' becomes 'roll' (rolling is a kind of functioning).
'roll' with its final letter removed is 'rol'.
'en'+'rol'='ENROL'
'of leaderless' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
(Other definitions for enrol that I've seen before include "Loner inclined to join in" , "Insert in register" , "Loner (anag.)" , "Bring into service" , "Register (as member)" .)