A word for saying things indirectly because you do not know the correct word

Solution 1:

Circumlocution

^{And by proxy: roundabout speech, circumduction, circumvolution, periphrasis, or ambage}

Roundabout speech refers to using many words (such as "a tool used for cutting things such as paper and hair") to describe something for which a concise (and commonly known) expression exists

^{source: wikipedia}

Solution 2:

Teachers will often ask their students, learners of English, to paraphrase the word or expression that they are stuck on, this might also include giving a definition of the said word.

Paraphrase

a rewording of something written or spoken by someone else.
Oxford dictionaries

Solution 3:

I agree with the commentator above that "circumlocution" has a connotation of intentionality. I would use "floundering" for the behavior originally described, although that might have more of a connotation of incoherence than intended by the original question.

Solution 4:

You can talk around the topic, as per this definition from Thefreedictionary.com:

_2. To speak indirectly about: talked around the subject but never got to the point.