Musician's desire to cover trio occasionally (6)
I believe the answer is:
lutist
'musician's' is the definition.
(I've seen this before)
'desire to cover trio occasionally' is the wordplay.
'desire' becomes 'lust' (lust is a kind of desire).
'to cover' indicates putting letters inside.
'occasionally' means one should take alternating letters (letters taken at regular intervals or occasions).
The alternate letters of 'trio' are 'ti'.
'lust' enclosing 'ti' is 'LUTIST'.
(Other definitions for lutist that I've seen before include "string player" , "musician" .)