Non-religious sector, popular with outsiders (7)
I believe the answer is:
secular
'non-religious' is the definition.
(I've seen this before)
I don't understand the remainder of the clue.
Can you help me to learn more?
(Other definitions for secular that I've seen before include "Worldly" , "Not connected with religion" , "Ignoring God" , "Not religious or sacred" , "associated with those who are 21 [LAYMEN]" .)