Against accepting new thought / idea, basically (4)
I believe the answer is:
anti
'against' is the definition.
(I've seen this before)
'accepting new thought idea basically' is the wordplay.
'basically' suggests taking the first letters (the first letter could be its root or base).
The first letters of 'accepting new thought idea' is 'ANTI'.
(Other definitions for anti that I've seen before include "Hostile" , "Opposed to followers of Hitler" , "Against, informally" , "Who pooh-poohs all" , "Prefix meaniing against" .)