Yes, knack is a Scrabble word!
...and is worth 15 points in Scrabble, and 17 points in Words with Friends
knack
Noun
1. A special way of doing something
What Scrabble words can I make with the letters in knack?
3 Letter Words
2 Letter Words
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Words that can be created with an extra letter added to knack:
S:knacks