How many letters does the top answer for “save up as for future use” have?

The most common answer is cache (5 letters).

Crossword clue: save up as for future use

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

cache 5 letters wn-def

a hidden storage space (for money or provisions or weapons)

stash 5 letters wn-def

a secret store of valuables or money