Former Test captain getting boundary, loudly cheer on (4,3)
I believe the answer is:
root for
'cheer on' is the definition.
(I've seen this in another clue)
'former test captain getting boundary loudly' is the wordplay.
'former test captain getting' becomes 'root' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'boundary' becomes 'four' (type of score in cricket).
'loudly' indicates a 'sounds like' (homophone) clue (when spoken - not silently).
'four' sounds like 'for'.
'root'+'for'='ROOT FOR'
Can you help me to learn more?
(Other definitions for root for that I've seen before include "cheer" , "support" .)