The most common and most likely answer to this crossword clue is the 7 letter word BASTARD
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Buddy explains!

The straight part
We know that "Fake!" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case fake! = bastard.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.
Crossword Answer definitions
BASTARD noun- A person who was born out of wedlock, and hence often considered an illegitimate descendant.
- A mongrel (biological cross between different breeds, groups or varieties).
- (typically referring to a man) A contemptible, inconsiderate, overly or arrogantly rude or spiteful person.
- A man, a fellow, a male friend.
- (often preceded by 'poor') A person deserving of pity.
- A child who does not know his or her father.
- Something extremely difficult or unpleasant to deal with.
- A variation that is not genuine; something irregular or inferior or of dubious origin, fake or counterfeit.
- An intermediate-grade file; also bastard file.
- A sweet wine.
- A sword that is midway in length between a short-sword and a long sword; also bastard sword.
- An inferior quality of soft brown sugar, obtained from syrups that have been boiled several times.
- A large mould for straining sugar.
- A writing paper of a particular size.
- A Eurosceptic Conservative MP, especially in the government of John Major.
- To bastardize.
- Of or like a bastard (illegitimate human descendant).
- Of or like a bastard (bad person).
- Of or like a mongrel, bastardized creature/cross.
- Of abnormal, irregular or otherwise inferior qualities (size, shape etc).
- Spurious, lacking authenticity: counterfeit, fake.
- Used in the vernacular name of a species to indicate that it is similar in some way to another species, often (but not always) one of another genus.
- Very unpleasant.
- Abbreviated, as the half title in a page preceding the full title page of a book.
- (theater lighting) Consisting of one predominant color blended with small amounts of complementary color; used to replicate natural light because of their warmer appearance.
- Exclamation of strong dismay or strong sense of being upset.