Stand-off is not quick rugby player (8)
I believe the answer is:
deadlock
'stand-off' is the definition.
(thesaurus)
'not quick rugby player' is the wordplay.
'not quick' becomes 'dead' (I've seen this before).
'rugby player' becomes 'lock' (second row of a scrum).
'dead'+'lock'='DEADLOCK'
'is' acts as a link.
(Other definitions for deadlock that I've seen before include "Impasse, standoff" , "tie" , "Intractable problem" , "Stalemate - it requires a key to open" , "Something opened by a key - stalemate" .)