Bar from behaving otherwise (5)
I believe the answer is:
ingot
'bar' is the definition.
(block of metal, eg a bar of gold)
'from behaving otherwise' is the wordplay.
'from' says the answer is hidden in the clue.
'INGOT' can be found hidden inside 'behaving otherwise'.
(Other definitions for ingot that I've seen before include "A brick shaped lump of gold" , "Block of, eg, gold" , "A lump of metal, perhaps gold" , "Bar, brick of metal" , "Bar, often gold" .)