The system will turn that clock on any time a peripheral needs it. I don't believe you can control it. The nRF51 documentation had a table showing which clocks each peripheral needed. I haven't seen a similar table for the nRF52 yet, but I haven't closely read the 52 documentation yet as we haven't migrated over to it yet.
EDIT: The PCLK32M is only used by two peripherals; the PDM and the I2S. It is not possible to drive other peripherals, like e.g. a timer, using the PCLK32M.
As John DeWitt says, there is a clock manager that starts the clock needed by a peripheral automatically. Here is a relevant answer.
You can start the HFCLK manually by writing a '1' to the CLOCK->TASKS_HFCLKSTART task. It will take some time to start the clock so you should wait for an EVENTS_HFCLKSTARTED event before allowing your application to proceed.
The table in the nRF51 documentation that John DeWitt is mentioning seems to have been forgotten in the nRF52 PS. I have reported it internally and hopefully we will see it included soon.
