If a phantom gets hit with an ion wepon can it still clock and declock?

No

When ioned, a ship does not get to reveal a maneuver dial, which is the trigger for decloaking.

However, an ioned ship can still cloak as being ioned does not prevent you from performing actions, and cloaking is an action.

Bottom of page 16 here:

http://www.fantasyflightgames.com/ffg_content/x-wing/support/faq/X-Wing-FAQ.pdf

Thanls guys

I know you didn't ask but might as well follow up with the flipside that a stressed ship cannot perform the cloak action but if a ship is already cloaked and gets a stress token, it can still decloak even when stressed.