Once again the Observer Everyman cryptic crossword provides some well-crafted clues which illustrate a few of the fiendish ways that expert compilers use to keep us entertained!
Observer Everyman 5/8/17
18D: Sincere, not caring? Not he. (7)
‘Not caring’ is HEARTLESS. Removing the letters HE gives the answer, a synonym of sincere.
8D: Structure for play with unknown actors in club one organised. (6, 6)
‘Structure for play’ is the definition. ‘Unknown’ is often used for X, Y or Z (as in an equation such as Y = X+2). Here it is Y and followed by CAST (ie ‘Actors’) inside an anagram of CLUB ONE. Note how the word ‘actors’ leads us down the garden path by suggesting a different meaning for ‘play’!