Googling
"frontal clause"
seems to bring up only references to 'full frontal clauses', relating to nudity in the theatre or on film.
Changing the search term to
"frontal clause" -full
(to get rid of all those nudity references) still doesn't bring up a single reference to a grammatical definition. I can only think that some college lecturer has decided to coin their own term for a clause (possibly adverbial) appearing at the start of a sentence.
Similarly, neither I nor Google has heard of an 'end clause', so I can only assume that it's being used to refer to a (possibly adverbial) clause at the end of a sentence.
A main (or independent) clause is a clause which can stand (as a complete sentence) in its own right.
A clause which adds something to a main (or independent) clause is a dependent clause. If the sentence would become ungrammatical without a particular dependent clause, that clause is then said to be 'embedded'.
Chris