Defender to spoil goat rotten (9)
I believe the answer is:
apologist
'defender' is the definition.
(I've seen this before)
'spoil goat rotten' is the wordplay.
'rotten' indicates anagramming the letters.
'spoil'+'goat'='spoilgoat'
'spoilgoat' is an anagram of 'APOLOGIST'.
'to' acts as a link.
(Other definitions for apologist that I've seen before include "Champion" , "Justifier" , "Defender of something controversial" , "One who argues in favour of something controversial" , "he puts up a defence" .)