A letter too hasty can be mere rubbish (5)
I believe the answer is:
trash
'mere rubbish' is the definition.
'trash' can be an answer for 'rubbish' (synonyms). I'm unsure of the 'mere' bit.
'a letter too hasty' is the wordplay.
'a letter' becomes 't' ().
'too hasty' becomes 'rash' (I've seen this in another clue).
't'+'rash'='TRASH'
'can be' is the link.
Can you help me to learn more?
(Other definitions for trash that I've seen before include "Take the tops from < rhubarb" , "Rubbish, waste" , "Refuse; wreck" , "Wreck; rubbish" , "Rubbish - destroy" .)