Do you know this word?

[Pentagon]

What 11-letter word means not being able to recall the word you want when speaking?

[Quizmonster]

Lethologica, apparently. Click here for more information.

[Peter Pedant]

not being able to name things is nominal dysphasia but i thought this almost certainly is not what you were thinking

[Quizmonster]

Dysphasia, like aphasia, is a condition brought on by brain damage, such as occurs in Alzheimer's Disease, for instance. I imagine the word Pentagon seeks is one which might refer to all of us when we simply have something on the tip of our tongues, but cannot temporarily access it.
As a matter of interest, scientists examining this phenomenon call it simply "a TOT experience", the 'TOT' standing for 'tip of tongue'.