Soldier takes gauge to be variable (9)
I believe the answer is:
parameter
'be variable' is the definition.
(parameter can mean a variable or value)
'soldier takes gauge' is the wordplay.
'soldier' becomes 'para' (paratrooper).
'takes' is a charade indicator (letters next to each other).
'gauge' becomes 'meter' (I've seen this in another clue).
'para'+'meter'='PARAMETER'
'to' acts as a link.
(Other definitions for parameter that I've seen before include "Factor defining limits" , "Limiting factor" , "Variable amount" , "Limitation" , "Variable allowed to change" .)