Ace rugby player, outstanding at the back, and fit (7)
I believe the answer is:
apropos
'fit' is the definition.
(thesaurus)
'ace rugby player outstanding' is the wordplay.
'ace' becomes 'a' (abbreviation).
'rugby player' becomes 'prop' (type of rugby player).
'outstanding' becomes 'os' (accounting abbreviation).
'a'+'prop'+'os'='APROPOS'
'at the back and' acts as a link.
This may not be right. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for apropos that I've seen before include "Poor sap, being to the point" , "suitably" , "In respect of" , "Poor sap (anag) - to the purpose" , "With reference to, concerning" .)