The most common and most likely answer to this crossword clue is the 8 letter word DIASTOLE
I've seen this crossword clue in The Daily Telegraph
D
I
A
S
T
O
L
E
Buddy explains!
The straight part
We know that "Bit of a heart-throb" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case bit of a heart-throb = diastole.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.
What does DIASTOLE mean?
DIASTOLE noun- The phase or process of relaxation and dilation of the heart chambers, between contractions, during which they fill with blood; an instance of the process.
- The lengthening of a vowel or syllable beyond its typical length.
- (Greek grammar) The hypodiastole, a textual or punctuation mark formerly used to disambiguate homonyms in Greek.