Politician, one defending goal, is tender of course (11)
I believe the answer is:
greenkeeper
'tender of course' is the definition.
(a greenkeeper tends a golf course)
'politician one defending goal' is the wordplay.
'politician' becomes 'green' (eg member of the Green Party).
'one defending goal' becomes 'keeper' (I can't justify this - if you can you should believe this answer much more).
'green'+'keeper'='GREENKEEPER'
'is' acts as a link.
Can you help me to learn more?
(Other definitions for greenkeeper that I've seen before include "Golf course maintainer" , "One looking after a golf course" , "Person caring for a golf course" , "One employed to look after a golf course" , "Person having care of golf course" .)