Calm, never flustered? Not so (9)
I believe the answer is:
nerveless
'calm' is the definition.
(I've seen this before)
'never flustered? not so' is the wordplay.
'flustered?' indicates anagramming the letters.
'not so' becomes 'less' (I've seen this before).
'never' anagrammed gives 'nerve'.
'nerve'+'less'='NERVELESS'
(Other definitions for nerveless that I've seen before include "no feeling" , "Not able to feel" , "Having great confidence" , "Showing calm self-control" , "calm and collected" .)