How many letters does the top answer for “a thief who breaks open safes to steal valuable contents” have?

The most common answer is cracksman (9 letters).

Crossword clue: a thief who breaks open safes to steal valuable contents

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

cracksman 9 letters wn-def

a thief who breaks open safes to steal valuable contents

safecracker 11 letters wn-def

a thief who breaks open safes to steal valuable contents