A clause is a unit of grammar that basically needs a subject and a predicate (I am sure you know both) and that enforces or enhances (as in the current question) the overall meaning of the sentence. As an example:
“I had to change my shirt, as it was wet!”
Here you can subdivide the sentence without breaking the rhythm of interpretation in two halves ( 1. I had to change my shirt… Nobody is interested here, why I had to change my shirt or what drove me to do that…, but the second half adds a fillip to the why (as it was wet…). That is the clause part.
Extending on this example, you have also the term modifier which usually involves either an adjective (that enhances a noun) or even an adverb (that change/adds to the verb in question, as the case may be). Once you qualify the section of the clause with either of these grammatical elements, there you got it!
1 I am in a lousy mood, leave me alone! (Here the modifier is lousy and it addresses the noun called mood).
2 I have a headache, please don’t talk so loudly! (Here the modifier is loudly and qualifies the verb talk).