Reed almost certainly jazzy (8)
I believe the answer is:
clarinet
'reed' is the definition.
(clarinet is a kind of reed)
'almost certainly jazzy' is the wordplay.
'almost' means to remove the last letter (most of the word but not all of it).
'jazzy' is an anagram indicator.
'certainly' with its last letter removed is 'certainl'.
'certainl' anagrammed gives 'CLARINET'.
(Other definitions for clarinet that I've seen before include "member of orchestra" , "Single-reed straight-tubed instrument" , "Wind instrument made from Clare tin" , "Music maker" , "Woodwind instrument played by Artie Shaw and Benny Goodman" .)