Delayed clunking into 1st gear---what's going on?

I have a 2000 4Runner with 81,000 miles. 5-speed manual. Lately it has intermittently failed to engage into first–I rev it and then it clunks into gear. Shifting into other gears not a problem–only from neutral into 1st.

Will it shift into first if the engine is not turning?

I know that the question sounds funny at first but it makes sense. If it shifts easily with the engine off then it may be a clutch problem, so, OP, please answer beadsandbeads’ question and he will work from there.