Possible Clues:

• Fir is
• Is there always a tree where you need a 1 across?
• One never leaves off
• Never leaves off, always retaining its vitality
• Still valid, if Cluedo suspect lacks beginning and end
• Not going brown, so maybe pine
• Leaves don't leave
• Streisand hit
• Never leaves off, but always starts 1 across
• Always envious of you, for example, so it's said