Exercises in pitch tail off (5)
I believe the answer is:
taper
'tail off' is the definition.
(I've seen this before)
'exercises in pitch' is the wordplay.
'exercises' becomes 'pe'.
'in' means one lot of letters goes inside another.
'pitch' becomes 'tar' ('tar' can be a synonym of 'pitch').
'pe' going inside 'tar' is 'TAPER'.
(Other definitions for taper that I've seen before include "Spill - become narrower towards one end" , "Peter out" , "Wind down" , "Diminish gradually" , "On VHS he recorded" .)