Bowl over, with very hot ball trapping opener in test (4)
I believe the answer is:
stun
'bowl over' is the definition.
(I've seen this before)
'trapping opener in test' is the wordplay.
'opener' suggests taking the first letters.
'in' is an insertion indicator.
'test' becomes 'sun' (I can't justify this - if you can you should give a lot more credence to this answer).
The initial letter of 'trapping' is 't'.
't' placed inside 'sun' is 'STUN'.
'with very hot ball' is the link.
This may not be right. Some or all of it may be part of another bit of the clue.
Can you help me to learn more?
(Other definitions for stun that I've seen before include "Render insensitive" , "Shock or astonish" , "Knock cold" , "Stagger, amaze" , "Confound" .)