Following behind king (5)
I believe the answer is:
later
'following' is the definition.
(thesaurus)
'behind king' is the wordplay.
'behind' becomes 'late'.
'king' becomes 'r' (r is abbreviation for rex).
'late'+'r'='LATER'
(Other definitions for later that I've seen before include "Afterward, subsequent" , "more modern" , "In due course" , "After a given point in time" , "Alert (anag.)" .)