The most common and most likely answer to this crossword clue is the 5 letter word CLYDE
C
L
Y
D
E
Buddy explains!
The straight part
We know that "Flower of Scotland — that runs close to Glasgow’s heart" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case flower of scotland — that runs close to glasgow’s heart = clyde.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.