Missionary position is used in Antilles regularly (7)
I believe the answer is:
apostle
'missionary' is the definition.
(I've seen this before)
'position is used in antilles regularly' is the wordplay.
'position' becomes 'pos' (I've seen this in other clues).
'is used in' means one lot of letters goes inside another.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
The alternating letters of 'antilles' are 'atle'.
'pos' going into 'atle' is 'APOSTLE'.
(Other definitions for apostle that I've seen before include "James, perhaps" , "Peter, perhaps" , "Chosen disciple" , "Vigorous campaigner" , "Leaps to find religious follower" .)