It's fundamental to the inner working? (8)
I believe the answer is:
inherent
'it's fundamental' is the definition.
(thesaurus)
'the inner working?' is the wordplay.
'working?' is an anagram indicator.
'the'+'inner'='theinner'
'theinner' is an anagram of 'INHERENT'.
'to' acts as a link.
(Other definitions for inherent that I've seen before include "deep-rooted" , "Underlying - integral" , "Integral, innate" , "Built-in, integral" , "Basic" .)