Creep hard to put in jail is a big hitter (12)
I believe the answer is:
sledgehammer
'a big hitter' is the definition.
(I've seen this before)
'creep hard to put in jail' is the wordplay.
'creep' becomes 'edge' (I've seen this in other clues).
'hard' becomes 'h'.
'to put in' indicates putting letters inside.
'jail' becomes 'slammer' (informal term for prison).
'edge'+'h'='edgeh'
'edgeh' going within 'slammer' is 'SLEDGEHAMMER'.
'is' acts as a link.
(Other definitions for sledgehammer that I've seen before include "One may be used to break rocks" , "Tool" , "Unsubtle" , "Big hitter" , "Unsuitable nutcracker?" .)