Without warning United captain is sacked (13)
I believe the answer is:
unanticipated
'without warning' is the definition.
(thesaurus)
'united captain is sacked' is the wordplay.
'is sacked' is an anagram indicator.
'united'+'captain'='unitedcaptain'
'unitedcaptain' is an anagram of 'UNANTICIPATED'.
(Other definitions for unanticipated that I've seen before include "Not expected" , "Not foreseen" .)