African country rejected adopting second religion (5)
I believe the answer is:
islam
'religion' is the definition.
(I know that Islam is a religion)
'african country rejected adopting second' is the wordplay.
'african country' becomes 'mali' (country in Africa).
'rejected' says the letters should be written in reverse.
'adopting' means one lot of letters goes inside another (inserted letters are taken on or adopted).
'second' becomes 's' (common abbreviation).
'mali' in reverse letter order is 'ilam'.
'ilam' going around 's' is 'ISLAM'.
(Other definitions for islam that I've seen before include "Monotheistic religion" , "Koran its scripture" , "Mohammed's religion" , "Faith with five pillars" , "Religion based on Muhammad's teachings" .)