Gem for disheartened rugby (4)
I believe the answer is:
ruby
'gem' is the definition.
(I know that ruby is a type of gem)
'disheartened rugby' is the wordplay.
'disheartened' suggests removing the centre.
'rugby' with its centre taken out is 'RUBY'.
'for' is the link.
(Other definitions for ruby that I've seen before include "Gemstone comprising mainly aluminium oxide" , "Deep red - transparent gemstone" , "Precious red gem" , "Vivid red" , "killed an assassin" .)