Either of the two tides that occur just after the first or third quarters of the moon (4)
I believe the answer is:
neap
(Other definitions for neap that I've seen before include "Tide when there is least difference between high and low water" , "A less than average tide" , "Tide when there is the least variation between high and low water" , "Tide sketched by a pen" , "Least in difference between high and low tides" .)