Sketches of runners going round finishing fast (5)
I believe the answer is:
skits
'sketches' is the definition.
(I've seen this before)
'runners going round finishing fast' is the wordplay.
'runners' becomes 'skis' (ski is a kind of runner).
'going round' means one lot of letters goes inside another.
'finishing fast' becomes 't' (I am not sure about this - if you are sure you should believe this answer much more).
'skis' going around 't' is 'SKITS'.
'of' is the link.
(Other definitions for skits that I've seen before include "Sketches" , "Satires" , "Short satirical pieces of drama" , "Pieces of humorous mimicry" .)