Logic behind Throckmorton's capital crime (7)
I believe the answer is:
treason
'crime' is the definition.
(treason is a kind of crime)
'logic behind throckmorton's capital' is the wordplay.
'logic' becomes 'reason' (I've seen this before).
'behind' means one lot of letters go next to another.
'capital' says to take the initial letters.
The initial letter of 'throckmorton' is 't'.
'reason' after 't' is 'TREASON'.
(Other definitions for treason that I've seen before include "Crime of disloyalty" , "LEse-majestE" , "Crime agains one's country" , "Gunpowder Plot, say" , "Criminal attempt to overthrow a government" .)