English king cutting public tax beyond what's reasonable (9)
I believe the answer is:
overexert
'tax beyond what's reasonable' is the definition.
(to tax or demand too much of someone)
'english king cutting public' is the wordplay.
'english' becomes 'E' (abbreviation).
'king' becomes 'rex' ('king' in Latin).
'cutting' means one lot of letters goes inside another (some letters cut their way into a word).
'public' becomes 'overt' (associated in meaning).
'e'+'rex'='erex'
'erex' inserted into 'overt' is 'OVEREXERT'.
(Other definitions for overexert that I've seen before include "Make too much effort" , "use too much" , "exercise unduly?" , "work too hard" , "strain" .)