Constant over constant is conclusively one, in short (7)
I believe the answer is:
concise
'short' is the definition.
(similar in meaning)
'constant over constant is conclusively one' is the wordplay.
'constant' becomes 'c' (mathematical abbreviation).
'over' becomes 'on' (physically on top of).
'constant' becomes 'c' (mathematical abbreviation).
'conclusively' indicates one should take the final letters (the letter which concludes the word).
The last letter of 'one' is 'e'.
'c'+'on'+'c'+'is'+'e'='CONCISE'
'in' acts as a link.
(Other definitions for concise that I've seen before include "Involving few words" , "Brief but comprehensive" , "Using few words" , "Compact - brief" , "Brief, said in few words" .)