Preposition or noun covered by tuition off and on (4)
I believe the answer is:
unto
'preposition' is the definition.
(I've seen this in another clue)
'noun covered by tuition off and on' is the wordplay.
'noun' becomes 'n'.
'covered by' indicates putting letters inside.
'off and on' indicates alternate letters (letters used at regular intervals).
The alternating letters of 'tuition' are 'uto'.
'n' inserted inside 'uto' is 'UNTO'.
'or' acts as a link.
(Other definitions for unto that I've seen before include "To old" , "To (obs.)" , "'The Lord said . . . . Moses'" , "In the direction of (archaic)" , "'Do ... others, etc.'" .)