Course we hear, is fixed (4)
I believe the answer is:
tied
'fixed' is the definition.
(tying is a kind of fixing)
'course we hear' is the wordplay.
'course' becomes 'tide' (tiding is a kind of coursing).
'we hear' shows a homophone (sound like).
'tide' sounds like 'TIED'.
'is' is the link.
(Other definitions for tied that I've seen before include "Restricted, bound" , "(Of pub) owned and run by a brewery" , "Made into a knot" , "eg Scored 1-1" , "With an equal score" .)