A bishop not entirely sharp? These assist with calculations (5)
I believe the answer is:
abaci
'these assist with calculations' is the definition.
(plural of abacus, early calculation device)
'a bishop not entirely sharp?' is the wordplay.
'bishop' becomes 'B' (abbreviation used in chess).
'not entirely' means to remove the last letter.
'sharp?' becomes 'acid' (as in sharp-tasting).
'acid' with its final letter taken away is 'aci'.
'a'+'b'+'aci'='ABACI'
(Other definitions for abaci that I've seen before include "They can be counted on" , "help for summer" , "frames" , "summers" , "on which people may be counting?" .)