Rock as tendons get damaged (9)
I believe the answer is:
sandstone
'rock' is the definition.
(sandstone is a kind of rock)
'as tendons get damaged' is the wordplay.
'get damaged' indicates an anagram.
'as'+'tendons'='astendons'
'astendons' is an anagram of 'SANDSTONE'.
(Other definitions for sandstone that I've seen before include "Type of sedimentary rock" , "Among the best of building materials" , "Rock containing quartz grains" , "Such building material" .)