The most common and most likely answer to this crossword clue is the 5 plus 3 plus 3 plus 3 letter word SALTY OLD SEA DOG
I've seen this crossword clue in Financial Times
S
A
L
T
Y
O
L
D
S
E
A
D
O
G
Buddy explains!
The straight part
We know that "chap in the third row" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case chap in the third row = salty old sea dog.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.