Follower absorbed in a system of rules (10)
I believe the answer is:
discipline
'a system of rules' is the definition.
(I know that discipline is a type of system)
'follower absorbed in' is the wordplay.
'follower' becomes 'disciple' (disciple is a kind of follower).
'absorbed' becomes 'in' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'in' is an insertion indicator.
'disciple' placed around 'in' is 'DISCIPLINE'.
Can you help me to learn more?
(Other definitions for discipline that I've seen before include "Correct" , "Training designed to engender self-control" , "Branch of knowledge; punish" , "Punishment" , "Correction" .)