Beeswax
He was given a clear warning. He ignored it and it cost him everything.
The across answers fill the grid in clue order, the first beginning in the top left corner and each one starting where the last finished. When a row runs out, the answer carries straight on at the start of the row below. The down answers do the same down the columns, the first again beginning in the top left corner, each carrying on at the top of the column to the right.
The three yellow squares hold multiple letters. Both answers crossing one use all of its letters, in order.
Fifteen answers come out one letter too long. Drop the extra letter before writing the answer in. Those fifteen dropped letters, in clue order, give a few words of advice that the person they were meant for should have taken.
Eight of the fifteen are down clues. The first letters of those eight, in clue order, give the man who gave the advice (8).
The number in brackets is the length of the word you solve, not of the shorter thing you write in. A few proper names turn up, all of them fair.