Santa worried about yours truly becoming a composer (7)
I believe the answer is:
smetana
'a composer' is the definition.
(I know that Smetana is a composer)
'santa worried about yours truly' is the wordplay.
'worried' indicates an anagram (worry can mean to interfere with or disturb).
'about' is an insertion indicator.
'yours truly' becomes 'me' (the person who set this crossword).
'santa' is an anagram of 'stana'.
'stana' enclosing 'me' is 'SMETANA'.
'becoming' is the link.
(Other definitions for smetana that I've seen before include "Czech composer, d. 1884 - Eastman (anag)" , "Composer of The Bartered Bride; eats man (anag.)" , "Nineteenth century Czech composer (The Bartered Bride)" , "Bedrich ____, composer of "The Bartered Bride" and "The Kiss"" , "maker of notes" .)