Is “Slacken” a Scrabble word?
Yes
SLACKEN
Valid in at least one Scrabble dictionary
Valid in these dictionaries
SOWPODS / CSW
Scrabble UK & International
TWL / NWL
Scrabble North America
Large English Dictionary
General English word list
Meaning of Slacken
/ˈslæ.kən/
verb
1.To gradually decrease in intensity or tautness; to become slack.
“The pace slackened.”
2.To make slack, less taut, or less intense.
3.To deprive of cohesion by combining chemically with water; to slake.
“to slacken lime”