One asks hard questions of horseman touring Donegal on vacation (7)
I believe the answer is:
riddler
'one asks hard questions' is the definition.
I don't know anything about this answer so I cannot tell whether it can be defined by this definition.
'horseman touring donegal on vacation' is the wordplay.
'horseman' becomes 'rider' (someone riding a horse).
'touring' indicates putting letters inside (touring can mean going round).
'on vacation' suggests removing the centre (after the inside letters have vacated the word).
'donegal' with its middle taken out is 'dl'.
'rider' going around 'dl' is 'RIDDLER'.
'of' is the link.
Can you help me to learn more?
(Other definitions for riddler that I've seen before include "Sphinx?" , "passing on only the finest material?" , "Puzzle setter" , "setter of puzzles" , "One straining" .)