Piece of canvas forming an extra covering over a tent (8)
I believe the answer is:
flysheet
'piece of canvas forming an extra' is the definition.
I know nothing about this answer so I can't judge whether it can be defined by this definition.
'covering over a tent' is the wordplay.
'covering' means one lot of letters go next to another (on top of in down clue).
'over' becomes 'sheet' (I can't justify this - if you can you should give a lot more credence to this answer).
'a tent' becomes 'fly' (fly tent is a kind of tent).
'sheet' put after 'fly' is 'FLYSHEET'.
Can you help me to learn more?
(Other definitions for flysheet that I've seen before include "Handbill - extra protection for a tent" , "extra protection when camping" .)