Secretly, only half of them support soldier (9)
I believe the answer is:
privately
'secretly' is the definition.
(synonyms)
'them support soldier' is the wordplay.
'them' becomes 'ly' (I can't explain this - if you can you should give a lot more credence to this answer).
'support' says to put letters next to each other (in a down clue, some letters hold others up).
'soldier' becomes 'private' (private is a type of soldier).
'ly' after 'private' is 'PRIVATELY'.
'only half of' acts as a link.
I am not very happy about this link. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for privately that I've seen before include "in camera?" , "Avoiding publicity" .)