Secondary PDA is an item slot that can be used by SPY. SPY has several options to use with this slot (each of them gives SPY a unique skill): Invis Watch, Enthusiast's Timepiece, Quäckenbirdt, Cloak and Dagger, and Dead Ringer. Find out the best price for TF2 Secondary PDA slots on DMarket.
On DMarket, you can also trade tf2 skins and get items that you really need.