Knock out boxer as everyone gulps (5)
I believe the answer is:
erase
'knock' is the definition.
The definition and answer can be both to do with contact as well as being verbs in their base form.
Maybe there's an association between them I don't understand?
'out boxer as everyone gulps' is the wordplay.
'out' indicates the answer is hidden within the clue (I've seen 'out of' mean this).
'ERASE' is hidden within 'boxer as everyone gulps'.
Can you help me to learn more?
(Other definitions for erase that I've seen before include "Remove evidence of" , "Remove all traces of" , "Rub out, cancel" , "Kill" , "Eradicate" .)