The right function of a device (4)
I believe the answer is:
ruse
'a device' is the definition.
(device can mean a scheme or ruse)
'the right function' is the wordplay.
'the right' becomes 'r' (common abbreviation).
'function' becomes 'use' (something's use is its function).
'r'+'use'='RUSE'
'of' is the link.
(Other definitions for ruse that I've seen before include "Sure to be a plot" , "Ploy, artifice" , "Plan intended to deceive" , "Deceptive manoeuvre" , "Subterfuge" .)