Show how old one is in a symptom of exhaustion (7)
I believe the answer is:
pageant
'show' is the definition.
(a show or display)
'how old one is in a symptom of exhaustion' is the wordplay.
'how old one is' becomes 'age'.
'in' is an insertion indicator.
'a symptom of exhaustion' becomes 'pant' (I am not sure about this - if you are sure you should believe this answer much more).
'age' placed into 'pant' is 'PAGEANT'.
Can you help me to learn more?
(Other definitions for pageant that I've seen before include "Make public spectacle" , "Parade with tableaux" , "Colourful procession" , "Neat gap for colourful spectacle" , "Spectacular display" .)