Period when hostilities, interrupting aid, end badly (9)
I believe the answer is:
edwardian
'period' is the definition.
(the Edwardian period)
'hostilities interrupting aid end badly' is the wordplay.
'hostilities' becomes 'war' (hostilities between two nations).
'interrupting' indicates putting letters inside.
'badly' indicates an anagram.
'aid'+'end'='aidend'
'aidend' anagrammed gives 'eddian'.
'war' put into 'eddian' is 'EDWARDIAN'.
'when' is the link.
(Other definitions for edwardian that I've seen before include "Of the fashion mocked by Teddy boys" , "After Victoria" , "from early twentieth century" , "The age before WW1" , "Early 20th-century" .)