Control what sounds like highly valued beast (8)
I believe the answer is:
reindeer
'beast' is the definition.
(I've seen this before)
'control what sounds like highly valued' is the wordplay.
'control' becomes 'rein' (synonyms).
'what sounds like' indicates a 'sounds like' (homophone) clue.
'highly valued' becomes 'dear' (expensive).
'dear' sounds like 'deer'.
'rein'+'deer'='REINDEER'
(Other definitions for reindeer that I've seen before include "Dancer's one" , "Arctic ruminant" , "Dancer, for one" , "Rudolph, say" , "Christmas card creatures" .)