Lies inert becoming buoyant (9)
I believe the answer is:
resilient
'buoyant' is the definition.
('resilient' can be a synonym of 'buoyant')
'lies inert becoming' is the wordplay.
'becoming' is an anagram indicator.
'lies'+'inert'='liesinert'
'liesinert' with letters rearranged gives 'RESILIENT'.
(Other definitions for resilient that I've seen before include "bouncing back" , "quick to bounce back" , "Irrepressible" , "Rebounding from impact" , "Hardy" .)