Bouncer initially tests England's captain (4)
I believe the answer is:
root
'england's captain' is the definition.
(Joe Root)
'bouncer initially tests' is the wordplay.
'bouncer' becomes 'roo' (informal term for a kangaroo).
'initially' says to take the initial letters.
The initial letter of 'tests' is 't'.
'roo'+'t'='ROOT'
(Other definitions for root that I've seen before include "Lowest part of plant" , "Part of plant under the ground" , "Underground part of a plant" , "Source; firmly establish" , "radical" .)