Yours truly, so fit, spinning the bottle (10)
I believe the answer is:
methuselah
'bottle' is the definition.
(I know that methuselah is a type of wine bottle)
'yours truly so fit spinning' is the wordplay.
'yours truly' becomes 'me' (the person that set this crossword).
'so' becomes 'thus' (synonyms).
'fit' becomes 'hale' (both can mean healthy).
'spinning' is a reversal indicator.
'hale' in reverse letter order is 'elah'.
'me'+'thus'+'elah'='METHUSELAH'
'the' acts as a link.
(Other definitions for methuselah that I've seen before include "One very old" , "a lot of bottle" , "Old man of the Bible" , "Large wine bottle - very long-lived man" , "I'm an old man" .)