One in my reserves is a very rich person (9)
I believe the answer is:
moneybags
'a very rich person' is the definition.
'one in my reserves' is the wordplay.
'in' indicates putting letters inside.
'reserves' becomes 'bags' (both can mean large amounts of something).
'one' going into 'my' is 'money'.
'money'+'bags'='MONEYBAGS'
'is' is the link.
(Other definitions for moneybags that I've seen before include "I enjoy lots of mint" , "Person who's loaded" .)