At first they insist musician is definitely retiring (5)
I believe the answer is:
timid
'retiring' is the definition.
(I've seen this in another clue)
'at first they insist musician is definitely' is the wordplay.
'at first' suggests taking the first letters.
The initial letters of 'they insist musician is definitely' is 'TIMID'.
(Other definitions for timid that I've seen before include "Inclined to avoid people" , "Bashful or fearful" , "Fearful - shy" , "Easily scared" , "Mouse" .)