Bothersome individuals - ones going into the finer details (9)
I believe the answer is:
nuisances
'bothersome individuals' is the definition.
(annoying or bothersome people)
'ones going into the finer details' is the wordplay.
'ones' becomes 'Is' (I is the Roman numeral for one).
'going into' is an insertion indicator.
'the finer details' becomes 'nuances' (nuances are finer details or complexities).
'is' put into 'nuances' is 'NUISANCES'.
(Other definitions for nuisances that I've seen before include "Annoyances, pains in the neck" , "problems" , "Pests or irritations" .)