King stopping gnome swimming in river (6)
I believe the answer is:
mekong
'river' is the definition.
(Mekong is an example)
'king stopping gnome swimming' is the wordplay.
'king' becomes 'k'.
'stopping' means one lot of letters goes inside another (stopping can mean blocking a gap).
'swimming' indicates anagramming the letters.
'gnome' is an anagram of 'meong'.
'k' going into 'meong' is 'MEKONG'.
'in' is the link.
(Other definitions for mekong that I've seen before include "Asian river, flows into South China Sea" , "River and delta in southern Vietnam" , "Asian banker" , "River with vast delta in Vietnam" , "The river flows through a large delta in Vietnam" .)