Finally decides to run off in a short, fast run (6)
I believe the answer is:
sprint
'a short fast run' is the definition.
(I've seen this before)
'finally decides to run off' is the wordplay.
'finally' indicates one should take the final letters.
'to run off' becomes 'print' (I've seen this in another clue).
The last letter of 'decides' is 's'.
's'+'print'='SPRINT'
'in' is the link.
(Other definitions for sprint that I've seen before include "Run quickly" , "Run rapidly" , "Short fast run" , "quick run" , "Short fast race" .)