Tide descriptor (4)
I believe the answer is:
neap
'tide descriptor' is the definition.
(I've seen this in another clue)
This is the entire clue.
(Other definitions for neap that I've seen before include "Small tide" , "Tide when there is least difference between high and low water" , "Tide of minimum range" , "Tide when there is the least variation between high and low water" , "A less than average tide" .)