I may bum around just like that time, getting muscle weakness (10)

'muscle weakness' is the definition.
The definition suggests a singular noun which matches the answer.

'i may bum around just like that time getting' is the wordplay.
'bum' indicates anagramming the letters (bum can mean faulty or wrong).
'around' indicates putting letters inside.
'just like' becomes 'shena' (I can't explain this - if you can you should give a lot more credence to this answer).
'that time' becomes 't' (abbreviation).
'getting' is an insertion indicator.
'shena' placed around 't' is 'sthena'.
'may' anagrammed gives 'mya'.
'mya'+'sthena'='myasthena'
'i' placed into 'myasthena' is 'MYASTHENIA'.

Can you help me to learn more?