Companion when young? He's too calm to get upset (10)
I believe the answer is:
schoolmate
'companion' is the definition.
(thesaurus)
'he's too calm to get upset' is the wordplay.
'to get upset' indicates an anagram (I've seen 'getting upset' mean this).
'hes'+'too'+'calm'='hestoocalm'
'hestoocalm' anagrammed gives 'SCHOOLMATE'.
'when young?' acts as a link.
I am not very happy about this link. Some or all of it may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for schoolmate that I've seen before include "Childhood friend" , "Friends Reunited contact" , "childhood chum" , "fishwife?" , "Fellow pupil" .)