Shows about how old the trousers are (8)
I believe the answer is:
pageants
'shows' is the definition.
'about how old the trousers are' is the wordplay.
'about' means one lot of letters goes inside another.
'how old' becomes 'age' (I've seen this before).
'the trousers are' becomes 'pants' (I can't justify this - if you can you should give a lot more credence to this answer).
'age' inserted within 'pants' is 'PAGEANTS'.
Can you help me to learn more?
(Other definitions for pageants that I've seen before include "spectacles" , "Elaborate processions or tableaux" , "Dramatic or spectacular tableaux" , "Rich and spectacular ceremonies" , "Costumed processions" .)