Shoal (4)
(Other definitions for lour that I've seen before include "(Of sky) look dark" , "Threatening appearance" , "Look sullen - frown" , "Scowl, look dark and threatening" , "Appear sullen or threatening" .)
'shoal' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for reef that I've seen before include "Great Barrier formation?" , "Ridge of rock just below ocean surface" , "Reduce a sail's surface - shipping hazard" , "Sandbank" , "portion of sail" .)
'shoal' is the definition.
(I know that shoal can be written as school)
This is all the clue.
(Other definitions for school that I've seen before include "Group of artists < in coach" , "Teach - fishes swimming" , "Forms" , "Where one teaches" , "Instruct" .)