The most common and most likely answer to this crossword clue is the 4 letter word PANG
P
A
N
G
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The straight part
We know that "ache" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case ache = pang.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 PANG mean?
PANG noun- (often in the plural) A paroxysm of extreme physical pain or anguish; a feeling of sudden and transitory agony; a throe.
- (often in the plural) A sudden sharp feeling of an emotional or mental nature, as of joy or sorrow.
- To cause to have great pain or suffering; to torment, to torture.