It's most desirable to enclose a wild animal (5)
I believe the answer is:
beast
'wild animal' is the definition.
(I've seen this before)
'it's most desirable to enclose a' is the wordplay.
'it's most desirable' becomes 'best' ('best' can be similar in meaning to 'most desirable').
'to enclose' indicates putting letters inside.
'best' enclosing 'a' is 'BEAST'.
(Other definitions for beast that I've seen before include "Pig" , "Animal, creature" , "Brute; creature" , "wild creature" , "Animal; cruel person" .)