Clothing design creates garments that may or may not be trendy.
Trend is based on community, on mass; when a majority considers something of a new/unique (design)style and want to associate themselves with it, you could see a trend appearing of appreciation for this certain style.
When a garment or style is in trend i.e. trendy, you could also say it is in fashion.
A trend being constant would mean it at some point becomes the norm to some extend. When a trend increases for it to degrease at a later stage, you could say it is a fashionable trend. For example; it has come into fashion to be associated with this trend (A), but trend A subsequently becomes surpassed by another trend (B), thus A is not able to sustain itself to become the new norm.
Fashion design therefor is a rather misguided term as it is not the designer that creates fashion, its the people that decide what is in fashion (which in tern is based on trend).
Something can be fashionable when it is designed to go along a certain trend that is currently in fashion. However, it is very unlikely that a designer creates something that will become fashionable, if it is not linked to a current trend within society. The term fashion designer therefor merely means that the designer is designing something that follows a currently fashionable trend (which either can be on the rise or decline, depending on the foresight of the designer). Success as a designer, or the success of their designs, is somewhat dependent on a variety they can bring within the boundaries of a current trend and on the suitability with a currently fashionable trend.