Grab one puzzle half-cracked by W (5,4)

'w' is the definition.
I don't know anything about this answer so I cannot judge whether it can be defined by this definition.

'grab one puzzle half-cracked' is the wordplay.
'grab' becomes 'sei' (I can't explain this - if you can you should believe this answer much more).
'half' means to take half the letters.
'cracked' indicates anagramming the letters.
'puzzle' with half the letters taken is 'puz'.
'one'+'puz'='onepuz'
'onepuz' is an anagram of 'zeupon'.
'sei'+'zeupon'='SEIZE UPON'

'by' is the link.

Can you help me to learn more?