Petty officer took tarnished shield and rubbed it (8)
I believe the answer is:
polished
'petty officer took tarnished' is the definition.
I can't judge whether this definition defines the answer.
'shield and rubbed it' is the wordplay.
I cannot really understand how this works, but
an anagram of 'shield' is 'lished' which is present in the answer.
The remaining letters 'po' is a valid word which might be clued in a way I don't see.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for polished that I've seen before include "Buffed to a shine" , "Shining or refined" , "Difference, statistical quantity" , "Shiny; rubbed" , "Made shiny and smooth" .)