It shows rank's a load of rubbish (6)
I believe the answer is:
stripe
'it' is the definition.
The definition suggests a singular noun which matches the answer.
'rank's a load of rubbish' is the wordplay.
'rank's' becomes 's' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'a load of rubbish' becomes 'tripe' (both can mean nonsense).
's'+'tripe'='STRIPE'
'shows' acts as a link.
Can you help me to learn more?
(Other definitions for stripe that I've seen before include "Narrow band of colour" , "Long band of a different colour" , "Sleeve badge showing military rank" , "Band of colour - 3 of rank" , "Band of colour, maybe of rank" .)