Mantis converted by morning prayers (6)
I believe the answer is:
matins
'morning prayers' is the definition.
(I've seen this before)
'mantis converted' is the wordplay.
'converted' is an anagram indicator.
'mantis' with letters rearranged gives 'MATINS'.
'by' acts as a link.
(Other definitions for matins that I've seen before include "C of E's daily morning service" , "Church service" , "Anglican morning service" , "Isn't ma given to morning prayers?" , "Morning prayers or bird-songs" .)