Turbulence in realm most unusual (9)
I believe the answer is:
maelstrom
'turbulence' is the definition.
(I have seen 'extreme turbulence' mean 'maelstrom' so perhaps 'turbulence' could also mean 'maelstrom')
'realm most unusual' is the wordplay.
'unusual' is an anagram indicator.
'realm'+'most'='realmmost'
'realmmost' anagrammed gives 'MAELSTROM'.
'in' acts as a link.
(Other definitions for maelstrom that I've seen before include "confused state of affairs" , "extreme turbulence" , "fearsome current" , "danger to shipping" , "Norwegian whirlpool" .)