Tabour beaten in modified rhythm (6)
I believe the answer is:
rubato
'modified rhythm' is the definition.
(I've seen this before)
'tabour beaten' is the wordplay.
'beaten' indicates an anagram.
'tabour' anagrammed gives 'RUBATO'.
'in' is the link.
(Other definitions for rubato that I've seen before include "time variation" , "Disregard for strict tempo in playing music" , "Expressively flexible tempo" , "regardless of tempo" , "Rhythmic flexibility" .)