First, we solve the cryptic clues. However, some of them don’t fit on the grid. The title clues that some of the words are A in common phrases in the form “A is B”. When we try entering B in the grid instead, everything works.

Across
  1. SILENCE is GOLDEN
  2. TIME is MONEY
  3. STRONGER
  4. PLUG
  5. WOE is ME
  6. SUCH is LIFE
  7. GLOBE
  8. DIE is CAST
  9. CELL
  10. BLOCK
  11. SCAR
  12. EX
  13. LESS is MORE
  14. SKY is THE LIMIT
  15. THROW
Down
  1. WRITING is ON THE WALL
  2. DROPLET
  3. NO GIF
  4. MORE
  5. NEPAL
  6. BABA is YOU
  7. SEEING is BELIEVING
  8. CURSIVE
  9. COAST is CLEAR
  10. TALK is CHEAP
  11. STEW
  12. JURY is OUT
Zigzag
  1. IGNORANCE is BLISS

We read the highlighted letters in order of number on the bottom right to get X IS WORD Y IS LETTER.