Weather rod in endless reef (5)
I believe the answer is:
erode
'weather' is the definition.
(I've seen this in another clue)
'rod in endless reef' is the wordplay.
'in' means one lot of letters goes inside another.
'endless' means to look at the middle letters.
The middle of 'reef' is 'ee'.
'rod' inserted into 'ee' is 'ERODE'.
(Other definitions for erode that I've seen before include "Gradually wear away" , "Wear away with weather" , "'Deteriorate, wear away (5)'" , "Abrade" , "Become ground down or deteriorate" .)