Time an exercise run, and tail off (5)
I believe the answer is:
taper
'tail off' is the definition.
(I've seen this before)
'time an exercise run' is the wordplay.
'time' becomes 't'.
'an' becomes 'a'.
'exercise' becomes 'pe' (Physical Education).
'run' becomes 'r' (cricket abbreviation).
't'+'a'+'pe'+'r'='TAPER'
'and' is the link.
(Other definitions for taper that I've seen before include "Get to the point" , "One making recording" , "Diminish gradually" , "person making recording" , "Recorder" .)