Second 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)
'second caber' is the wordplay.
'second' becomes 's' (common abbreviation).
'caber' becomes 'log' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
's'+'log'='SLOG'
Can you help me to learn more?
(Other definitions for slog that I've seen before include "Exhausting task" , "almighty wallop" , "Work (or walk) involving great effort" , "Hit hard - hard trek" , "Long tedious task" .)