Appreciate the referee showing restraint (6)
I believe the answer is:
tether
'restraint' is the definition.
(tether is a kind of restraint)
'appreciate the referee showing' is the wordplay.
'showing' indicates the answer is hidden within the clue.
'TETHER' is hidden in the letters of 'appreciate the referee'.
(Other definitions for tether that I've seen before include "Restrictive rope fixed to one spot" , "Rope to fasten or restrain animal" , "Rope for tying animals" , "Rope for confining animal" , "Rope holding an animal" .)