Tool gives advantage to Henry in jail (12)
I believe the answer is:
sledgehammer
'tool' is the definition.
(sledgehammer is a kind of tool)
'advantage to henry in jail' is the wordplay.
'advantage' becomes 'edge' (having the edge is having an advantage).
'to henry' becomes 'H' (symbol for the unit of electrical inductance).
'in' indicates putting letters inside.
'jail' becomes 'slammer' (informal term for prison).
'edge'+'h'='edgeh'
'edgeh' going into 'slammer' is 'SLEDGEHAMMER'.
'gives' is the link.
(Other definitions for sledgehammer that I've seen before include "Unsubtle" , "Dealer of heavy blow" , "OTT version of the Nutcracker?" , "Unsuitable nutcracker?" , "One may be used to break rocks" .)