Small caber is hard work (4)
I believe the answer is:
slog
'is hard work' is the definition.
(a slog is a long or difficult task)
'small caber' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'caber' becomes 'log' (I can't explain this - if you can you should believe this answer much more).
's'+'log'='SLOG'
Can you help me to learn more?
(Other definitions for slog that I've seen before include "Wild cricket hit" , "Hit hard - hard trek" , "Walk hard carrying logs?" , "Work (or walk) involving great effort" , "big hit" .)