Tube may damage ulna (7)
I believe the answer is:
cannula
'tube' is the definition.
(cannula is a kind of tube)
'may damage ulna' is the wordplay.
'may' becomes 'can' ('can' can be a synonym of 'may').
'damage' indicates anagramming the letters (I've seen this in other clues).
'ulna' is an anagram of 'nula'.
'can'+'nula'='CANNULA'
(Other definitions for cannula that I've seen before include "Thin tube inserted into a vein to deliver medication" , "Small tube inserted into the body to give medication" , "Small flexible tube inserted into a body for medication" .)