Spanish golfer next to tree plays regularly and badly (8)
I believe the answer is:
severely
'badly' is the definition.
(I know that badly can be written as severely)
'spanish golfer next to tree plays regularly' is the wordplay.
'spanish golfer' becomes 'Seve' (Seve Ballesteros).
'next to' says to put letters next to each other.
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
'tree'+'plays'='treeplays'
The alternating letters of 'treeplays' are 'rely'.
'seve'+'rely'='SEVERELY'
'and' is the link.
(Other definitions for severely that I've seen before include "Strictly, harshly" , "With sternness" , "Intensely; harshly" , "very badly" , "Sternly or gravely" .)