The Trim Whitespace item is available via the Reformat Text button on the collection toolbar, except when the Recycle Bin tab is active.
Select the Trim Whitespace command or press Ctrl+T on the keyboard to remove any whitespace from the start and the end of the active clip's text. Spaces, tabs and line breaks are all considered whitespace.
When selecting text in a document or a web page, it takes quite a bit of effort to make sure no superfluous whitespace characters end up in the selection. When using AceText, you don't have to worry about this. Just capture the text into AceText, and press Ctrl+T to get rid of the additional whitespace, if any.
To remove extraneous whitespace at the start and the end of each line, use Trim Whitespace from Paragraphs instead of or in addition to Trim Whitespace.
See Editing Clips and Text to learn how to use AceText.