Secretly chummy with soldier (2,7)
I believe the answer is:
in private
'secretly' is the definition.
(I've seen this before)
'chummy with soldier' is the wordplay.
'chummy' becomes 'in' (in the sense of being 'in' with a particular crowd).
'with' is a charade indicator (letters next to each other).
'soldier' becomes 'private' (I've seen this before**).
'in'+'private'='IN PRIVATE'
(Other definitions for in private that I've seen before include "Confidentially" , "behind closed doors" , "secretly" , "behind the scenes" .)