Natural gas in neither fluid (8)
I believe the answer is:
inherent
'natural' is the definition.
(I've seen this before)
'gas in neither fluid' is the wordplay.
'gas' becomes 'N' (chemical symbol for Nitrogen).
'in' is an insertion indicator.
'fluid' indicates an anagram.
'neither' anagrammed gives 'iherent'.
'n' inserted within 'iherent' is 'INHERENT'.
(Other definitions for inherent that I've seen before include "constitutional" , "Integral, innate" , "Integral, inbuilt" , "Underlying - integral" , "Built-in, integral" .)