One, thick-skinned, who could become richer soon? (10)
I believe the answer is:
rhinoceros
'one thick-skinned' is the definition.
(thick-skinned animal)
'become richer soon?' is the wordplay.
'become' indicates an anagram.
'richer'+'soon'='richersoon'
'richersoon' with letters rearranged gives 'RHINOCEROS'.
'who could' acts as a link.
(Other definitions for rhinoceros that I've seen before include "Big game" , "Thick-skinned beast" , "Pachyderm" , "Brute" , "Ionesco play" .)