An ulna broken yearly? (6)
I believe the answer is:
annual
'yearly?' is the definition.
('annual' can be a synonym of 'yearly')
'an ulna broken' is the wordplay.
'broken' indicates anagramming the letters.
'ulna' is an anagram of 'nual'.
'an'+'nual'='ANNUAL'
(Other definitions for annual that I've seen before include "Happening once each year" , "Reference book that's published every year" , "Occurring yearly" , "Book; occurring once a year" , "Regular publication" .)