Poirot's old labourer (8)
I believe the answer is:
hercules
'labourer' is the definition.
(I've seen this before)
'poirot's old' is the wordplay.
'poirot' becomes 'hercule' (Hercule Poirot).
'old' becomes 's' (I can't explain this - if you can you should give a lot more credence to this answer).
'hercule'+'s'='HERCULES'
Can you help me to learn more?
(Other definitions for hercules that I've seen before include "His labours are legendary" , "Mythological hero" , "Fabled performer of labours" , "Hero of Greek myth who performed 12 great labours" , "Northern constellation" .)