It slithers along, negotiating ledges (6)
I believe the answer is:
sledge
'it slithers along' is the definition.
The definition suggests a singular noun which matches the answer.
'negotiating ledges' is the wordplay.
'negotiating' is an anagram indicator (I've seen 'negotiated' mean this).
'ledges' with letters rearranged gives 'SLEDGE'.
Can you help me to learn more?
(Other definitions for sledge that I've seen before include "Vehicle for use in snow" , "Vehicle for travelling over snow" , "In Canada train" , "Toboggan" , "Large heavy hammer" .)