I've copied this one:
Clue: Saw dog wearing lead (7)
(Wearing is the containment indicator in the clue.)
To solve this requires "Rover" for dog to be placed inside "lead" in this case the metal's abbreviation pb. This gives proverb which is another word for Saw which was what the setter was after.
I hope this explains what a containment indicator is. I'm relatively new to all this stuff myself. Someone else might do it better. There are loads of these indicator words for anagrams, when to insert,when to delete,when to use first and last letter to name but a few. This Chambers Crossword Dictionary which I have has helped me quite a bit.
Andy