The most common and most likely answer to this crossword clue is the 9 letter word RIGHTY-HO
I've seen this crossword clue in The Guardian
R
I
G
H
T
Y
-
H
O
Buddy explains!
The straight part
We know that "will do" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case will do = righty-ho.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 RIGHTY-HO mean?
RIGHTY-HO interjection- To indicate agreement
- (introducing a sentence in order to draw attention to the importance of what is being said) alright, well (then)