Highly valued ruminant animal discussed (4)
I believe the answer is:
dear
'highly valued' is the definition.
(expensive)
'ruminant animal discussed' is the wordplay.
'ruminant animal' becomes 'deer'.
'discussed' shows a homophone (sound like).
'deer' is a homophone of 'DEAR'.
(Other definitions for dear that I've seen before include "At a high cost" , "pet > ducks" , "Baby" , "polite form of address" , "letter opener" .)