The most common and most likely answer to this crossword clue is the 8 letter word ANNOYING
I've seen this crossword clue in The Times
A
N
N
O
Y
I
N
G
More clues leading to the result ANNOYING
- It's galling, using PA bar first
- Tiresome nanny and I go out
- Irritating girl playing around, wasting time
- Tiresome nanny I almost got sacked
- Writhing in agony, holding front of nose, becoming tiresome
- Any gin? No? Awfully tiresome
- Vexatious refusal adopted by some in Government at first
- Troublesome girl, losing head, flirting lightly
- Doctor in agony about terminal condition that's troublesome
Buddy explains!

The straight part
We know that "Tiresome" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case tiresome = annoying.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 ANNOYING mean?
ANNOYING verb- To disturb or irritate, especially by continued or repeated acts; to bother with unpleasant deeds.
- To do something to upset or anger someone; to be troublesome.
- To molest; to harm; to injure.
- That which annoys; an annoyance.
- Causing irritation or annoyance; troublesome; vexatious.