Phlegmatic type almost shut in thus (5)
I believe the answer is:
stoic
'phlegmatic type' is the definition.
(I've seen this before)
'almost shut in thus' is the wordplay.
'almost shut' becomes 'to' (I can't explain this - if you can you should believe this answer much more).
'in' means one lot of letters goes inside another.
'thus' becomes 'sic' ('sic' can be a synonym of 'thus').
'to' put within 'sic' is 'STOIC'.
(Other definitions for stoic that I've seen before include "Zeno, say" , "Ancient philosopher" , "Seeming unaffected by pleasure or pain" , "Greek in school" , "One showing endurance during adversity" .)