That rider tumbling is mediocre (5-4)
I believe the answer is:
third-rate
'mediocre' is the definition.
(thesaurus)
'that rider tumbling' is the subsidiary indication.
'tumbling' indicates an anagram.
'that'+'rider'='thatrider'
'thatrider' anagrammed gives 'thirdrate'.
'is' is the link.
(Other definitions for third-rate that I've seen before include "Of very poor quality" , "Very inferior in quality in numeric description" , "Very inferior in numerical way" , "Very inferior in quality, using ordinal number" , "Very inferior in quality - count down" .)