Philosophical sort almost shut in thus (5)
I believe the answer is:
stoic
'philosophical' is the definition.
(thesaurus)
'shut in thus' is the wordplay.
'shut' becomes 'to' (I can't justify this - if you can you should give a lot more credence to this answer).
'in' is an insertion indicator.
'thus' becomes 'sic' ('thus' in Latin).
'to' placed into 'sic' is 'STOIC'.
'sort almost' acts as a link.
This may not be correct. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for stoic that I've seen before include "Solemn ceremony" , "Person indifferent to pleasure or pain" , "Impassive type" , "philosophical type" , "Ancient philosopher" .)