Grass forming part of meat-free diet (4)
I believe the answer is:
reed
'grass' is the definition.
(I've seen this before)
'part of meat-free diet' is the wordplay.
'part of' indicates the answer is hidden within the clue.
'REED' is hidden within 'meat free diet'.
'forming' acts as a link.
(Other definitions for reed that I've seen before include "A blade of march grass" , "Tall grass of wet areas" , "Tall stiff marsh or water grass" , "piece for organ" , "Oliver, say" .)