Substance that’s primarily sustaining assorted plants (3)
I believe the answer is:
sap
'substance' is the definition.
(sap is a kind of substance)
'primarily sustaining assorted plants' is the wordplay.
'primarily' suggests taking the first letters.
The initial letters of 'sustaining assorted plants' is 'SAP'.
'that's' acts as a link.
(Other definitions for sap that I've seen before include "Cosh; plant's juice" , "dig tunnels" , "Fluid in plant stems" , "It's plant juice, idiot" , "Drain of energy" .)