In sound condition though possibly highly strung (3,2,1,6)
I believe the answer is:
fit as a fiddle
'condition though possibly highly strung' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.
'in sound' is the wordplay.
I cannot really understand how this works, but
'sound' could be 'fit' (associated in meaning) and 'fit' is found in the answer.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for fit as a fiddle that I've seen before include "What item must do to go into violin case" , "Healthy (like a violin?)" , "In very good health" , "Bouncing with health" , "Cheerful and well" .)