Tidal adjective (4)
I believe the answer is:
neap
'tidal adjective' 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 "Tide with the smallest vertical range" , "The half-way kind of tide" , "Least in difference between high and low tides" , "A less than average tide" , "Tide sketched by a pen" .)