Quite forgetting to count (5)
I believe the answer is:
tally
'count' is the definition.
(I know that count can be written as tally)
'quite forgetting to' is the wordplay.
'quite' becomes 'totally' (quite can mean totally or completely).
'forgetting' suggests deleting specific letters.
'totally' with 'to' removed is 'TALLY'.
(Other definitions for tally that I've seen before include "Record; current score" , "Record; agree" , "highly?" , "Count, add up" , "Mark" .)