Best to accept new response, all the same (9)
I believe the answer is:
identical
'all the same' is the definition.
(thesaurus)
'best to accept new response' is the wordplay.
'best' becomes 'ideal' (similar in meaning).
'to accept' indicates putting letters inside.
'new' becomes 'n' (common abbreviation eg NT for New Testament).
'response' becomes 'tic' (I have seen 'Involuntary response ' mean 'tic' so perhaps 'response' could also mean 'tic').
'n'+'tic'='ntic'
'ideal' going around 'ntic' is 'IDENTICAL'.
(Other definitions for identical that I've seen before include "Like two peas in a pod" , "lacking distinction" , "Matching" , "Not any different" , "uniform" .)