When the water moving ain't deep (4,4)
I believe the answer is:
neap tide
'when the water' is the definition.
I can't tell whether this defines the answer.
'moving ain't deep' is the wordplay.
'moving' is an anagram indicator.
'aint'+'deep'='aintdeep'
'aintdeep' with letters rearranged gives 'NEAP TIDE'.
Can you help me to learn more?
(Other definitions for neap tide that I've seen before include "Deep drop" , "I tape end (anag.)" , "It follows the first or third quarters of the moon" , "Less than average rise and fall of the sea level" , "Smallest range in movement of sea" .)