Mark Fox turns up (3)
I believe the answer is:
dot
'mark' is the definition.
(dotting is a kind of marking)
'fox turns up' is the wordplay.
'fox' becomes 'tod' (historic term).
'turns up' is a reversal indicator.
'tod' in reverse letter order is 'DOT'.
(Other definitions for dot that I've seen before include "A point, small round mark" , "Little girl" , "'Spot, tiny mark (3)'" , "scatter" , "One signal used in Morse code" .)