The most competent stable reorganisation (6)
I believe the answer is:
ablest
'the most competent' is the definition.
(I've seen this before)
'stable reorganisation' is the wordplay.
'reorganisation' indicates an anagram.
'stable' with letters rearranged gives 'ABLEST'.
(Other definitions for ablest that I've seen before include "Most proficient" , "Most capable" , "Most competent in stable" , "Like pupils in top stream" , "Most suitable" .)