Lamb's utterance about keeping king's horse (4)
I believe the answer is:
arab
'horse' is the definition.
(Arab is a type of horse)
'lamb's utterance about keeping king's' is the wordplay.
'lamb's utterance' becomes 'baa' (sheep go baa).
'about' shows that the letters should be reversed in order.
'keeping' indicates putting letters inside (some letters kept inside).
'king' becomes 'R' (abbreviation of Latin rex).
'baa' back-to-front is 'aab'.
'aab' going around 'r' is 'ARAB'.
(Other definitions for arab that I've seen before include "Saudi, e.g." , "Fine horse breed" , "Mount" , "Horse of a breed originating in the Middle East" , "Person from the Middle East, say" .)