No thanks required by count, last off, so don't mention it (3,2,3)
I believe the answer is:
not at all
'don't mention it' is the definition.
(I've seen this before)
'no thanks required by count last off' is the wordplay.
'thanks' becomes 'ta' (UK informal version of 'thank you').
'required by' means one lot of letters go next to another.
'count' becomes 'tally' ('tally' can be a synonym of 'count').
'last off' means to remove the last letter.
'tally' with its last letter taken away is 'tall'.
'no'+'ta'+'tall'='NOT AT ALL'
'so' is the link.
(Other definitions for not at all that I've seen before include "acceptance of apology" , "Anything but!" , "In no way" , "My pleasure" , "Don't mention it!" .)