Thanks Roof,
4d - The "board" in question relates to a game and the "mass" is a weight.
23d - "strapping" is a containment indicator, so you have "by" going around/strapping an "American" - definition is "tied up"
25a - I'd never heard of this phrase and needed a wordsearch engine to get it. See Johnnykingsman's post @8 for the parsing.
PS Thank you JohnnyKingsman, for explaining the wordplay:-)