Reason for using an abacus? Absolutely (7)
I believe the answer is:
totally
'absolutely' is the definition.
(synonyms)
'reason for using an abacus?' is the wordplay.
I cannot quite understand how this works, but
'for' could be 'to' (eg both can mean 'in the opinion of') and 'to' is present in the answer.
'an' could be 'a' and 'a' is present in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for totally that I've seen before include "Wholly" , "Altogether" , "Completely, entirely" , "Quite" , "without reservation" .)