Well i'm talking out of experience, clue scrolls are frustrating sometimes, but when you get a good one, depending on what lvl clue you get, you can make either a decent amount or a ton of cash.
Here's my couple best that i can remember: Zammy Cape, Strength Ammy (T), Ammy of Glory (T), green dhide chaps and body (G), a ton of Zammy, Saradomin, and Guthix book pages, Rune plate (Zammy), a guthix coif, and a ton of rune stuff from lvl 3's.
These are what i kill mostly for clues. In order of Monster, then Clue Level Dropped.
Guards (Any City) - Level 2 clues
Daggonaths (LightHouse Basement) - Level 2 Clues
Green Dragons ("Wildy") - Level 3 Clues
Blue Dragons (Taverly Dungeon, Ranging, Maging, or Hally-ing) - Level 3 Clues
Hellhounds (Taverly Dungeon, Maging, Ranging, or Hally-ing) - Level 3 Clues
Some good items you can get and what level clue you can get them from.
Robin Hood Hat - Level 2 (Very Rare)
Ranger Boots - Level 2 (Very Rare)
3rd Age, Anything - Level 3 - (Extremely Rare)
Zammy, Saradomin, Guthix Capes - Level 2's and 3's
Zammy, Saradomin, Guthix Mitres - Level 2's
Zammy, Saradomin, Guthix Robes - Level 2's and 3's
Green/Blue Dhide Chaps and Body (G) - Level 2's for Green, Level 3's for Blue
Trimmed Armor (Rune Plate (G), (T), (Z), (G), (S) for example) - Level 1's for Black armor, Level 2's for Addy Armor, and Level 3's for Rune armor.
Ammy of Glory, Strength, Mage (T) - Level 2's for Strength and Mage, and Level 3's for Glory
Zammy, Saradomin, Guthix Book Pages - Level 2's and 3's
And a TON of other stuff you can get.
Good Luck Gettin a good clue