Rugby player extremely eager to be polite (6)
I believe the answer is:
proper
'be polite' is the definition.
(thesaurus)
'rugby player extremely eager' is the wordplay.
'rugby player' becomes 'prop' (I've seen this in other clues).
'extremely' suggests removing the centre.
'eager' with its middle removed is 'er'.
'prop'+'er'='PROPER'
'to' is the link.
(Other definitions for proper that I've seen before include "Appropriate or decorous" , "Genuine; correct" , "Conventional" , "Appropriate according to rules" , "Right or suitable" .)