Synthetic protein ultimately only fake (5)
I believe the answer is:
nylon
'synthetic' is the definition.
(synthetic material)
'protein ultimately only fake' is the wordplay.
'ultimately' says to take the final letters.
'fake' indicates an anagram.
The final letter of 'protein' is 'n'.
'only' anagrammed gives 'ylon'.
'n'+'ylon'='NYLON'
(Other definitions for nylon that I've seen before include "Synthetic material used in textiles" , "A thermoplastic" , "Man-made fibre" , "Fibre of polymeric amides" , "Synthetic polymer" .)