Fantastic hit with castle running a part of this (9)
I believe the answer is:
athletics
'running a part of this' is the definition.
The definition suggests a singular noun which matches the answer.
'fantastic hit with castle' is the wordplay.
'fantastic' indicates anagramming the letters (a fantastic or improbable ordering of the letters).
'with' means one lot of letters go next to another.
'hit' after 'castle' is 'castlehit'.
'castlehit' anagrammed gives 'ATHLETICS'.
Can you help me to learn more?
(Other definitions for athletics that I've seen before include "on the track, perhaps" , "Lithe acts observed in sporting contests" , "Olympic events" , "Physical sports for trained competitors" , "Running and jumping" .)