Soldier coming down with device to measure boundary (9)
I believe the answer is:
parameter
'measure boundary' is the definition.
'parameter' can be an answer for 'boundary' (I've seen this in another clue). I am not sure about the 'measure' bit.
'soldier coming down with device' is the wordplay.
'soldier coming down' becomes 'para' (short for paratrooper).
'with' says to put letters next to each other.
'device' becomes 'meter' (meter is a kind of device).
'para'+'meter'='PARAMETER'
'to' is the link.
Can you help me to learn more?
(Other definitions for parameter that I've seen before include "Variable amount" , "Factor defining limits" , "Variable allowed to change" , "One of a set of factors as conditions for operation of a system" , "Limiting factor" .)