20a - a 2 letter synonym for 'as', and the 3rd word adjustment leads to a 3 letter synonym (its last letter moving to the front of the answer as a whole).
38a - 4th word adjustment leading to 3 letter synonym [reversed after...].
21d - 5th word adjustment - subtractive anagram (minus - 1 man).
Now got a full grid, and when I shade the thematic bits I get a number for the top LH cell. It's the final instruction (8,3,7,2,5,2,7,2,7) that I need to sort out.