A chap's followed on infiltrating cult, one working undercover (6,5)
I believe the answer is:
secret agent
'one working undercover' is the definition.
(a secret agent works undercover)
'a chap's followed on infiltrating cult' is the wordplay.
'a chap' becomes 'agent' (I am not sure about this - if you are sure you should believe this answer much more).
'followed' is a charade indicator (letters next to each other) (some letters go after others).
'on' becomes 're' (regarding).
'infiltrating' is an insertion indicator.
'cult' becomes 'sect' (religious sect).
're' put into 'sect' is 'secret'.
'agent' after 'secret' is 'SECRET AGENT'.
Can you help me to learn more?
(Other definitions for secret agent that I've seen before include "Conrad character" , "Spy" , "007 - centre stage (anag)" .)