Tree trunks are terrible slog (4)
I believe the answer is:
logs
'tree trunks' is the definition.
The definition and answer can be both man-made objects as well as being plural nouns.
Perhaps there's a link between them I don't understand?
'terrible slog' is the wordplay.
'terrible' indicates anagramming the letters.
'slog' is an anagram of 'LOGS'.
'are' is the link.
Can you help me to learn more?
(Other definitions for logs that I've seen before include "Firewood; records" , "Written records of events on board" , "Written record of a voyage" .)