Fitting to include soldier with American instruments (9)
I believe the answer is:
apparatus
'instruments' is the definition.
(apparatus means pieces of equipment)
'fitting to include soldier with american' is the wordplay.
'fitting' becomes 'apt' (I've seen this before).
'to include' indicates putting letters inside.
'soldier with' becomes 'para' (short for paratrouper. I am not sure about the 'with' bit.).
'american' becomes 'us' (abbreviation for United States).
'apt' placed around 'para' is 'apparat'.
'apparat'+'us'='APPARATUS'
(Other definitions for apparatus that I've seen before include "Group of parts that work together to perform a function" , "Equipment, device" , "system" , "Collection of equipment designed for a particular purpose" , "Equipment designed for a specific function" .)