Set in nothing, a plant (7)
I believe the answer is:
nigella
'plant' is the definition.
(nigella is a kind of plant)
'set in nothing a' is the wordplay.
'set' becomes 'gel' (both can mean to harden).
'in' indicates putting letters inside.
'nothing' becomes 'nil' (eg nil is zero in football).
'gel' going inside 'nil' is 'nigell'.
'nigell'+'a'='NIGELLA'
(Other definitions for nigella that I've seen before include "TV personality" , "plant" , "Flower and TV chef" , "Flower and tv cook" , "Woman" .)