Phlegmatic master in Devon, extremely upset when disturbed (15)

I believe the answer is:

undemonstrative

'phlegmatic' is the definition.
(thesaurus)

'master in devon extremely upset when disturbed' is the wordplay.
'extremely' suggests removing the centre (only the extremes of the word are used).
'when disturbed' indicates an anagram.
'upset' with its middle taken out is 'ut'.
'master'+'in'+'devon'+'ut'='masterindevonut'
'masterindevonut' anagrammed gives 'UNDEMONSTRATIVE'.

(Another definition for undemonstrative that I've seen is " Cold".)

I've seen this clue in The Times.

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