Calculation aid (5,8)
I believe the answer is:
ready reckoner
'calculation aid' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for ready reckoner that I've seen before include "Calculation aid" , "A whiz at mental arithmetic" , "table" , "Tables of standard numerical calculations" , "I've done your sums for you" .)