Do fat cells function as an endocrine gland?

Yes

Explanation:

Fat cells occur mainly under the skin, performing functions of energy reserve, protection against mechanical shocks and thermal insulation. It also occurs around some organs such as the kidneys and the heart.

Endocrine glands are those that release their secretions (hormones) directly into the bloodstream or lymphatic vessels.

Fat cells can act as an endocrine gland because, by accumulating an amount of fat, these cells begin to produce lipin which is a hormone that acts on the hypothalamus and causes appetite to decrease.