Old computer in decline, on order (5,4)
I believe the answer is:
slide rule
'old computer' is the definition.
(I've seen this before)
'decline on order' is the wordplay.
'decline' becomes 'slide' (I've seen this in another clue).
'on order' becomes 'rule' (I've seen this in another clue. I am not sure about the 'on' bit.).
'slide'+'rule'='SLIDE-RULE'
'in' is the link.
Can you help me to learn more?
(Other definitions for slide rule that I've seen before include "Instrument for rapid calculations" , "Old calculator" , "Old calculating device" , "Obsolete calculator" , "old computer" .)