Encountered a stair, breaking bones (9)
I believe the answer is:
metatarsi
'bones' is the definition.
(I've seen this before)
'encountered a stair breaking' is the wordplay.
'encountered' becomes 'met' ('meet' can be a synonym of 'encounter').
'breaking' indicates anagramming the letters.
'stair' is an anagram of 'tarsi'.
'met'+'a'+'tarsi'='METATARSI'
(Other definitions for metatarsi that I've seen before include "Parts of the feet" , "bones" .)