It set up a test on extremely hardy grass (7)
I believe the answer is:
timothy
'grass' is the definition.
(I know that timothy is a type of grass)
'it set up a test on extremely hardy' is the wordplay.
'set up' shows that the letters should be reversed in order (in a down clue, letters go up).
'a test' becomes 'MOT' (as in an MOT test for a vehicle).
'on' says to put letters next to each other.
'extremely' means to remove the middle letters (only the extremes of the word are used).
'hardy' with its middle taken out is 'hy'.
'it' backwards is 'ti'.
'ti'+'mot'+'hy'='TIMOTHY'
(Other definitions for timothy that I've seen before include "Biblical writer" , "A grass; two NT books" , "Man's name" , "New Testament book" , "haymaker?" .)