It's attached to a trunk (4)
I believe the answer is:
limb
'trunk' is the definition.
The answer and definition can be both body parts as well as being singular nouns.
Maybe there's an association between them I don't understand?
I don't understand the rest of the clue.
Can you help me to learn more?
(Other definitions for limb that I've seen before include "Branch - arm - leg" , "as part of plane, say" , "Human's extension" , "Substantial branch from a tree trunk" , "Part of body or branch of tree" .)