Rules about a lot of whip leather (7)
I believe the answer is:
cowhide
'leather' is the definition.
(cowhide is a kind of leather)
'rules about a lot of whip' is the wordplay.
'rules' becomes 'code'.
'about' indicates putting letters inside.
'a lot of' means to remove the last letter (most of the word but not all of it).
'whip' with its last letter removed is 'whi'.
'code' enclosing 'whi' is 'COWHIDE'.
(Other definitions for cowhide that I've seen before include "whip" , "neat wrapper?" , "Type of leather" , "Giving leather" .)