Way of working out made up indoors (5,8)
I believe the answer is:
modus operandi
'way of working' is the definition.
'out made up indoors' is the wordplay.
'out' indicates anagramming the letters.
'made'+'up'+'indoors'='madeupindoors'
'madeupindoors' anagrammed gives 'MODUS OPERANDI'.
(Other definitions for modus operandi that I've seen before include "Habitual procedure" , "this shows how it's done" , "Particular way of doing something (Latin)" , "Classically typical style" , "Method of working" .)