Percussionist's effect is diverting, full of spirit (4,4)
I believe the answer is:
drum roll
'percussionist's effect' is the definition.
(effect played by a percussionist)
'diverting full of spirit' is the wordplay.
'diverting' becomes 'droll' (I've seen this in another clue).
'full of' means one lot of letters goes inside another.
'spirit' becomes 'rum' (alcoholic spirit).
'droll' placed around 'rum' is 'DRUM ROLL'.
'is' acts as a link.
(Other definitions for drum roll that I've seen before include "musical introduction?" , "Tattoo" , "Haydn symphony" , "Rapid percussive sound" .)