Note mum keeping one strong (9)
I believe the answer is:
resilient
'strong' is the definition.
'note mum keeping one' is the wordplay.
'note' becomes 're' (as in do-re-mi).
'mum' becomes 'silent' (to keep mum can informally mean to stay silent).
'keeping' means one lot of letters goes inside another.
'one' becomes 'i' (Roman numeral).
'silent' placed around 'i' is 'silient'.
're'+'silient'='RESILIENT'
(Other definitions for resilient that I've seen before include "bouncing back" , "quick to bounce back" , "Able to recover quickly" , "Bouncy" , "Able to bounce back from adversity" .)