The bit on the end is a mere morsel (4)
I believe the answer is:
bite
'mere morsel' is the definition.
'bite' can be an answer for 'morsel' (synonyms). I'm not sure about the 'mere' bit.
'the bit on the end' is the wordplay.
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'end' suggests the final letters.
The final letter of 'the' is 'e'.
'bit'+'e'='BITE'
'is a' acts as a link.
Can you help me to learn more?
(Other definitions for bite that I've seen before include "champ" , "Take effect; snack" , "Grip with teeth" , "Small amount of food" , "Use one's teeth" .)