Keen to trash the joint (4)
I believe the answer is:
knee
'the joint' is the definition.
(knee is a kind of joint)
'keen to trash' is the wordplay.
'to trash' indicates an anagram.
'keen' is an anagram of 'KNEE'.
(Other definitions for knee that I've seen before include "Flexible joint" , "for going on begging" , "middle part of leg" , "Genuflector?" , "joint input" .)