The most common and most likely answer to this crossword clue is the 5 letter word LARGE
I've seen this crossword clue in The Guardian
L
A
R
G
E
More clues leading to the result LARGE
- Injured leg broken by car without front bumper
- Very big poplar gets cut down
- Big canal boat bachelor put aside for learner
- Generous initial contributions from London and region greatly esteemed
- . Big drink with Resistance Movement
- Big league auctioneer’s covering empty garage
- Substantial part of secular generation
- Extensive extract from popular genealogy
- Big boat left for Belgium
- Whacking cane, flipping clever idea, pupil finally bending over
Buddy explains!

The straight part
We know that "Whacking" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case whacking = large.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 LARGE mean?
LARGE noun- An old musical note, equal to two longas, four breves, or eight semibreves.
- Liberality, generosity.
- (plural: large) A thousand dollars/pounds.
- A large serving of something.
- Of considerable or relatively great size or extent.
- Abundant; ample.
- Full in statement; diffuse; profuse.
- Free; unencumbered.
- Unrestrained by decorum; said of language.
- Crossing the line of a ship's course in a favorable direction; said of the wind when it is abeam, or between the beam and the quarter.
- Before the wind.