Creature that's a sort mostly protected by benefit (8)
I believe the answer is:
platypus
'creature' is the definition.
(platypus is a kind of creature)
'a sort mostly protected by benefit' is the wordplay.
'sort' becomes 'type' (type is a kind of sort).
'mostly' means to remove the last letter.
'protected by' means one lot of letters goes inside another.
'benefit' becomes 'plus' (both can mean an advantageous feature).
'a'+'type'='atype'
'atype' with its final letter removed is 'atyp'.
'atyp' placed inside 'plus' is 'PLATYPUS'.
'that's' is the link.
(Other definitions for platypus that I've seen before include "Australian resident" , "Aquatic creature" , "river creature" , "Primitive mammal, duck-billed" , "Australian egg-producer" .)