Salt-free diets must include grass (4)
I believe the answer is:
reed
'grass' is the definition.
(I've seen this before)
'salt-free diets must include' is the wordplay.
'must include' indicates a hidden word.
'REED' can be found hidden inside 'salt free diets'.
(Other definitions for reed that I've seen before include "Stalk" , "One of two in a 25 [OBOE]" , "blown in the wind" , "Grassy plant of wet areas" , "Marsh plant with tall hollow stem" .)