All but one of the team drop out of this (5,4)
I believe the answer is:
relay race
'drop out of this' is the definition.
The definition suggests a singular noun which matches the answer.
'all but one of the team' is the wordplay.
I cannot really see how this works, but
'one' could be 'ace' (ace has a value of one in some card games) and 'ace' is found in the answer.
'team' could be 'relay' (I have seen 'Team of workers' mean 'relay' so perhaps 'team' could also mean 'relay') and 'relay' is present in the answer.
A single letter 'r' remains which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
(Other definitions for relay race that I've seen before include "Foot speed contest passing batons" , "Baton-passing event?" , "Ace rarely placed in team event" , "having started, one couldn't finish it" , "Team contest" .)