Last month a porter [carrying a basket of tomatoes in the crowded Shasha market in Ibadan, a city in south-western Nigeria],
accidentally spilled his cargo, [leaving a pulpy mess].
The main/matrix (independent) clause here is the sentence as a whole. It is not dependent on some other element.
The bracketed "carrying" clause is modifying the head noun “porter” and hence is subordinate (dependent).
The bracketed "leaving" clause is an adjunct in clause structure, and thus subordinate (dependent).
The expression “accidentally spilled his cargo, leaving a pulpy mess” is a verb phrase functioning as the predicate of the main clause (the sentence).