Material X subsumes fellow, oddly (6)
I believe the answer is:
teflon
'material' is the definition.
(synthetic resin)
'x subsumes fellow oddly' is the wordplay.
'x' becomes 'ten' (synonyms).
'subsumes' means one lot of letters goes inside another (subsume can mean to absorb).
'oddly' indicates alternate letters (letters in the odd-numbered positions are used).
The alternate letters of 'fellow' are 'flo'.
'ten' going around 'flo' is 'TEFLON'.
(Other definitions for teflon that I've seen before include "Non-stick coating for pots and pans" , "Synthetic resin to prevent sticking" , "Non-stick synthetic resin" , "Non-stick coating - evading blame" , "Non-stick coating as for frying pans" .)