Calm about the terrible cost of getting out of line (5)
I believe the answer is:
stoic
'calm' is the definition.
(thesaurus)
'about the terrible cost of getting out of line' is the wordplay.
'about' indicates putting letters inside.
'the terrible' is an anagram indicator.
'of' becomes 'o'.
'getting out of' indicates named letters should be taken away.
'line' becomes 'oi' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'cost' with 'o' taken away is 'cst'.
'cst' anagrammed gives 'stc'.
'stc' placed around 'oi' is 'STOIC'.
Can you help me to learn more?
(Other definitions for stoic that I've seen before include "he's not complaining" , "Seeming unaffected by pleasure or pain" , "Solemn ceremony" , "One indifferent to fortune, pleasure and pain" , "One patient under misfortune" .)