Inner wall of the ditch holds even less fish (6)
I believe the answer is:
escarp
'inner wall of the ditch holds' is the definition.
I can't judge whether this definition defines the answer.
'even less fish' is the wordplay.
'even' means one should take alternating letters (even-numbered letters are used).
'fish' becomes 'carp' (carp is a kind of fish).
The alternate letters of 'less' are 'es'.
'es'+'carp'='ESCARP'
Can you help me to learn more?
(Other definitions for escarp that I've seen before include "Steep artificial slope in front of a fortification" , "part of fortification" , "Inner side of ditch in fortifications" , "Slope beneath rampart" , "Steep rise once" .)