Soldier coming down to record boundary (9)
I believe the answer is:
parameter
'boundary' is the definition.
'soldier coming down to record' is the wordplay.
'soldier coming down' becomes 'para' (short for paratrooper).
'to record' becomes 'meter' (meter can mean to measure with a meter).
'para'+'meter'='PARAMETER'
(Other definitions for parameter that I've seen before include "Variable amount" , "One of a set of factors as conditions for operation of a system" , "Limiting factor" , "Variable allowed to change" , "Factor defining limits" .)