To put it concisely, confined to a lunatic's inferno (2,1,8)
I believe the answer is:
in a nutshell
'to put it concisely confined' is the definition.
I can't tell whether this defines the answer.
'a lunatic's inferno' is the wordplay.
I cannot quite understand how this works, but
'a' could be 'an' and 'an' is found in the answer.
'inferno' could be 'hell' (synonyms) and 'hell' is found within the answer.
'lunatic' could be 'nut' (I've seen this before) and 'nut' is present in the answer.
The remaining letters 'is' is a valid word which might be clued in a way I don't understand.
This may be the basis of clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for in a nutshell that I've seen before include "Succinctly" , "To cut a long story short" , "Expressed briefly" , "Briefly, concisely" , "Neatly summarised" .)