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 am not certain 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" .)