As afterthought, after a moment one straightens elbow (7)
I believe the answer is:
triceps
'elbow' is the definition.
The definition and answer can be both body parts as well as being singular nouns.
Perhaps there's a link between them I don't understand?
'as afterthought after a moment one straightens' is the wordplay.
I cannot really see how this works, but
'afterthought' could be 'ps' (ps stands for postscript) and 'ps' is found in the answer.
'moment' could be 'trice' (I've seen this in another clue) and 'trice' is found in the answer.
No letters remain.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for triceps that I've seen before include "One stretches out an arm!" , "Green citrus fruit" , "Three-headed muscle" , "one can't bend the elbow!" , "Muscles" .)