Twice-monthly tide (4)
I believe the answer is:
neap
'twice-monthly tide' 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 "Least in difference between high and low tides" , "Tide when there is the least variation between high and low water" , "A less than average tide" , "The half-way kind of tide" , "Sort of tide" .)