Has occasion to sneer inwardly over this setter (5)
I believe the answer is:
needs
'has occasion' is the definition.
'needs' can be an answer for 'has' ('need' can be a synonym of 'have'). I am not certain of the 'occasion' bit.
'sneer inwardly over this setter' is the wordplay.
'inwardly' says to take the centre (only the letters inside the word are used).
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'this setter' becomes 'ds' (I can't explain this - if you can you should believe this answer much more).
The middle letters of 'sneer' are 'nee'.
'nee'+'ds'='NEEDS'
'to' acts as a link.
Can you help me to learn more?
(Other definitions for needs that I've seen before include "Essentials" , "Can't do without" , "Things that are wanted" , "Demands" , "shopping-list, say" .)