SB 7.11.17

jaghanyo nottamāṁ vṛttim
 anāpadi bhajen naraḥ
ṛte rājanyam āpatsu
 sarveṣām api sarvaśaḥ
Word for word: 
jaghanyaḥ — low (person); na — not; uttamām — high; vṛttim — means of livelihood; anāpadi — when there is no social upheaval; bhajet — may accept; naraḥ — a man; ṛte — except; rājanyam — the profession of the kṣatriyas; āpatsu — at times of emergency; sarveṣām — of everyone in every status of life; api — certainly; sarvaśaḥ — all professions or occupational duties.
Translation: 
Except in a time of emergency, lower persons should not accept the occupational duties of those who are higher. When there is such an emergency, of course, everyone but the kṣatriya may accept the means of livelihood of others.
Purport: 

The occupational duty of a brāhmaṇa should not be accepted by persons in lower social orders, especially vaiśyas and śūdras. For example, an occupational duty of the brāhmaṇa is to teach Vedic knowledge, but unless there is an emergency, this professional duty should not be accepted by the kṣatriyas, vaiśyas or śūdras. Even a kṣatriya cannot accept the duties of a brāhmaṇa unless there is an emergency, and then even if he does so he should not accept charity from anyone else. Sometimes brāhmaṇas protest against our Kṛṣṇa consciousness movement for creating brāhmaṇas from Europeans, or, in other words, from mlecchas and yavanas. This movement, however, is here supported in Śrīmad-Bhāgavatam. At the present moment, society is in a chaotic condition, and everyone has given up the cultivation of spiritual life, which is especially meant for the brāhmaṇas. Because spiritual culture has been stopped all over the world, there is now an emergency, and therefore it is now time to train those who are considered lower and condemned, so that they may become brāhmaṇas and take up the work of spiritual progress. The spiritual progress of human society has been stopped, and this should be considered an emergency. Here is solid support from Nārada Muni of the movement known as Kṛṣṇa consciousness.