King following traveller on water (6)
I believe the answer is:
rafter
'traveller on water' is the definition.
(someone travelling with a raft)
'king following' is the wordplay.
'king' becomes 'R' (abbreviation for rex, king in Latin).
'following' becomes 'after' (I've seen this before).
'r'+'after'='RAFTER'
(Other definitions for rafter that I've seen before include "Sloping roof timber" , "Sloping beam supporting a roof" , "Strong wooden beam for roof" , "support" , "traveller on log platform" .)