Chap is terrible bore about river temperature (6)
'chap' is the definition.
'terrible bore about river temperature' is the wordplay.
'terrible' indicates anagramming the letters.
'about' indicates putting letters inside.
'river' becomes 'r'.
'temperature' becomes 't' (abbreviation).
'bore' is an anagram of 'ober'.
'ober' going within 'rt' is 'ROBERT'.
'is' is the link.
(Other definitions for robert that I've seen before include "Victorian premier", "Boy", "- Browning, Eng. poet", "-- Browning, nineteenth century poet", "-- Browning, English poet".)