Half-moon tide (4)
I believe the answer is:
neap
'half-moon 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 "Small tide" , "A less than average tide" , "Tide with least fall and rise of water" , "Sort of tide" , "It's between spring and ebb" .)