Almost torn up as mere rubbish (3)
I believe the answer is:
rot
'mere rubbish' is the definition.
'rot' can be an answer for 'rubbish' (both can mean nonsense). I'm unsure of the 'mere' bit.
'almost torn up' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'up' is a reversal indicator (in down clue: letters go upwards).
'torn' with its final letter taken off is 'tor'.
'tor' back-to-front is 'ROT'.
'as' is the link.
Can you help me to learn more?
(Other definitions for rot that I've seen before include "Break down, waste away" , "Twaddle" , "'Decay, decompose (3)'" , "Rhubarb" , "Decay in grotesque way" .)