Nonet arranged as a mass (5)
I believe the answer is:
tonne
'a mass' is the definition.
(I've seen this before)
'nonet arranged' is the wordplay.
'arranged' is an anagram indicator (letters rearranged).
'nonet' with letters rearranged gives 'TONNE'.
'as' acts as a link.
(Other definitions for tonne that I've seen before include "1000 kilograms" , "A thousand kilogrammes" , "One thousand kilograms" , "Great weight" , "Metric weight unit" .)