Son doing physical exercises — overdoing them? (9)
I believe the answer is:
straining
'son' is the definition.
Although both the answer and definition are singular nouns, I cannot understand how one could define the other.
'doing physical exercises overdoing them?' is the wordplay.
I cannot really see how this works, but
'doing' could be 'strain' (strain is a kind of doing) and 'strain' is located in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for straining that I've seen before include "Making a great effort" , "Exerting much effort or energy" , "demanding" .)