I have finished this (I think), but am away from my Chambers and/or unable to parse some of the answers (conjectured letters given in lowercase below).
11d. One thing plucked inside another, joined at a stroke. HyPHEnAtE ?
13d. Earthy deposits creating meander, as before, in river, English. TeRrAMAzE ?
18d. Crudity delineated when scratching just the opening latter. RAwNeSS ?
Of course, any of these guesses may be wrong, or based on wrong crossing lights !
Neveracrossword, Mamalynne, and Bibblebub, thank you all!
• I am afraid my catalogue of Greek lyre strings is missing a few pages! But th definition seems fuzzy; shouldn't it be "join" rather than "joined at a stroke"?
• Terramare is new to me, and I still can't see the function of "as before" in the parsing.
• I thought "drawn" would somehow be part of it, but missed the "ess."
TERRAMARE - Chambers gives 'wander' as an archaic meaning of ERR, hence the 'as before' (Azed will always indicate archaic or obsolete words/meanings).
HYPHENATE - is also an adjective, meaning 'hyphenated', so the definition is fine.