Temporary-employment agencies benefit not only from the increasing demand for clerical workers but also from the higher profits made in placing highly paid professionals, requests for whom have increased in the recent wave of corporate takeovers.
This is a sentence from GMAT test, and it is a right sentence, but I think it is a run-on sentence(S+V,S+V). Can any one help me to figure out what structure it is.
There is nothing wrong with the sentence. It is not a run-on sentence. The second half of the sentence is a relative clause (with its own verb) used to provide information about the "professionals". The comma is included because the clause is not required to identify the "professionals" - it just adds additional information. Had it been required to identify them, then there would have been no comma.
e.g. ...but also from the higher profits made in placing highly paid professionals with whom a contract has been agreed.
(i.e. not those who do not have a contract.)
This is covered in Grammar Monster here.