Metal cover that protects the sewer (7)
I believe the answer is:
thimble
'cover that protects the sewer' is the definition.
'thimble' can be an answer for 'cover' (I've seen this before). I'm unsure of the rest of the definition.
'metal' is the wordplay.
I cannot really understand how this works, but
'metal' could be 'th' (Th is an example) and 'th' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for thimble that I've seen before include "Protective item for the tailor" , "Caucus-race prize (Alice)" , "Digital protection" , "Cap used for protection when sewing" , "See 5" .)