Regularly enhance reef with pearly stuff (5)
I believe the answer is:
nacre
'pearly stuff' is the definition.
(I've seen this in another clue)
'regularly enhance reef' is the wordplay.
'regularly' means one should take alternating letters (regularly take one letter, leave next etc.).
'enhance'+'reef'='enhancereef'
The alternate letters of 'enhancereef' are 'NACRE'.
'with' is the link.
(Other definitions for nacre that I've seen before include "inside of shell" , "valuable material" , "Mother of pearl" , "Constituent of inlay furniture" , "Mother-of-pearl - crane (anag)" .)