Material natural originally, only modified (5)
I believe the answer is:
nylon
'material' is the definition.
(nylon is a kind of material)
'natural originally only modified' is the wordplay.
'originally' suggests taking the first letters.
'modified' is an anagram indicator.
The first letter of 'natural' is 'n'.
'only' anagrammed gives 'ylon'.
'n'+'ylon'='NYLON'
(Other definitions for nylon that I've seen before include "Synthetic cloth" , "Man-made material" , "Fabric (originally from New York and London)" , "may be damaged by running" , "Man-made fibre" .)