How many letters does the top answer for “cook or heat in a microwave oven” have?

The most common answer is nuke (4 letters).

Crossword clue: cook or heat in a microwave oven

2 answers found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

nuke 4 letters wn-def

the warhead of a missile designed to deliver an atom bomb

zap 3 letters wn-def

a sudden event that imparts energy or excitement, usually with a dramatic impact

cook with dry heat, usually in an oven 50% similar → roast