Find undressed squaddie behind closed doors (2,7)
I believe the answer is:
in private
'behind closed doors' is the definition.
(I've seen this before)
'find undressed squaddie' is the wordplay.
'undressed' means to look at the middle letters (the word removes the letters which clothe it).
'squaddie' becomes 'private'.
The centre of 'find' is 'in'.
'in'+'private'='IN PRIVATE'
(Other definitions for in private that I've seen before include "secretly" , "behind closed doors" , "behind the scenes" , "Confidentially" .)