Airborne soldier invested in suitable American kit (9)
I believe the answer is:
apparatus
'kit' is the definition.
(I've seen this before)
'airborne soldier invested in suitable american' is the wordplay.
'airborne soldier' becomes 'para' (I've seen this before).
'invested in' means one lot of letters goes inside another.
'suitable' becomes 'apt' (I've seen this before).
'american' becomes 'us' (abbreviation for United States).
'para' going inside 'apt' is 'apparat'.
'apparat'+'us'='APPARATUS'
(Other definitions for apparatus that I've seen before include "Collection of equipment designed for a particular purpose" , "Equipment designed for a specific function" , "Technical equipment" , "Tackle" , "Equipment, gear" .)