Baseball player's clothing covers up grass (9)
I believe the answer is:
shortstop
'baseball player's' is the definition.
(shortstop is a kind of baseball player)
'clothing covers up grass' is the wordplay.
'clothing' becomes 'shorts' (shorts is a kind of clothing).
'covers' means one lot of letters go next to another.
'up' says the letters should be written in reverse (in down clue: letters go upwards).
'grass' becomes 'pot' (cannabis).
'pot' backwards is 'top'.
'shorts'+'top'='SHORTSTOP'
(Other definitions for shortstop that I've seen before include "Fielder in baseball" , "One in field" , "Baseball player" , "US player" , "position in American sport" .)