Cook's left Endeavour exhausted - it's dark and often bitter (6)

I haven't cracked this one yet — but the brilliant team at Fifteensquared have:

PORT (left in nautical terminology, so for example Captain James Cook’s word for left) + E [ndeavou] R (exhausted = inner letters emptied out). The reference to HMS Endeavour is perhaps enough of a clue to suggest this particular Cook. Porter = a term for dark beer.