Leaving bash held back in old country (6)
I believe the answer is:
exodus
'leaving' is the definition.
(thesaurus)
'bash held back in old country' is the wordplay.
'bash' becomes 'do' (synonyms).
'held back' shows that the letters should be reversed in order.
'in' indicates putting letters inside.
'old' becomes 'ex' (prefix meaning former or onetime).
'country' becomes 'US' (United States).
'do' back-to-front is 'od'.
'ex'+'us'='exus'
'od' put into 'exus' is 'EXODUS'.
(Other definitions for exodus that I've seen before include "Mass evacuation" , "Book following Genesis" , "8, Book 2" , "Great departure of people like Jews from Egypt" , "Journey of escape" .)