Nasty mark on end of toe is a big worry (5)
I believe the answer is:
scare
'a big worry' is the definition.
The answer and definition can be both feelings as well as being singular nouns.
Perhaps you can see a link between them that I don't see?
'nasty mark on end of toe' is the wordplay.
'nasty mark' becomes 'scar' (I've seen this before).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'end of' suggests the final letters.
The final letter of 'toe' is 'e'.
'scar'+'e'='SCARE'
'is' is the link.
Can you help me to learn more?
(Other definitions for scare that I've seen before include "Health worry" , "Cause fright, alarm" , "Startle" , "Frighten, alarm" , "Greatly frighten" .)