Plant, toxic ultimately, that hurt one playing in field (7)
I believe the answer is:
cowslip
'plant' is the definition.
(cowslip is a kind of plant)
'toxic ultimately that hurt one playing in field' is the wordplay.
'ultimately' indicates one should take the final letters.
'that hurt' becomes 'ow' (exclamation of pain).
'one playing in field' becomes 'slip' (I can't justify this - if you can you should believe this answer much more).
The final letter of 'toxic' is 'c'.
'c'+'ow'+'slip'='COWSLIP'
Can you help me to learn more?
(Other definitions for cowslip that I've seen before include "Yellow flower for unsteady bovine?" , "Flower related to primrose" , "Plant with yellow flower" , "Fictional rabbit" , "Pretty little wild flower for unsteady bovine" .)