DP went back and did that 'How a . . .' thing. Many clues were suggestive of the answer without being precise. The surface reading of clues was bizarre at times.
7) DECLINE AND FALL : decline,and,fall : the definition is half a phrase really. Not sure it stands by itself. There is a Evelyn Waugh book of that title.
9) HERON : --her on--
10) HEAD FIRST : head,first : maybe half a tumble.
11) TWISTER : ddef : nothing good to see here. Move along.
12) HECTARE : (the,race)* : for a simple clue, DP could've worked the surface a bit better.
13) TOSS : to,SS : if you took "How to" away, nothing changes, so why have it ?
14) SCANT : s----,can't
15) CLUB : ddef-ish
19) EXPORTS : ex,ports
21) EQUINOX : equin-,ox : a misuse of 'daylong' and a bizarre surface. Tripe.
22) OUT OF FORM : ddef-ish : an apt answer these days.
24) TASKS : St< <>ask
25) FIRST-CLASS MAIL : 1st Class mail