Insect repellent clothing can do the job in one of two ways...
Permethrin is a synthetic pyrethroid insecticide which kills insects if they ingest it and an insect repellent which deters insects from landing on you before biting. Read more on insect repellents for Africa.
The most common proprietary brands of insect repellent used in the manufacture of adventure clothing are Insect Shield and Nosilife.
They both work in much the same way - bonding a permethrin formula to the garment fibres which mask the human odours that are so attractive to insects. The pre-treated garments are odourless and protect against mosquitoes, ticks, fleas and flies. They are said to 'permanently' retain the repellent properties at least for 70 washes or the natural life of the garment. Just do not dry clean the item.
Bear in mind only the skin surface covered with the permethrin-treated clothing is protected from insects. Other exposed parts of the body need a skin applied insect repellent for more complete protection.
You are out of luck here...No insecticide or repellent, on clothes or not, seem to work to reliably deter the tsetse fly and nothing has changed since this 1989 study Evaluations of permethrin-impregnated clothing and three topical repellent formulations of deet against tsetse flies in Zambia. So the best that can be expected from permethrin impregnated clothing is 34% reduction of tsetse fly bites for the untreated and exposed skin of head and hands.
When the tsetse flies are really bad, only olive-coloured, permethrin-treated mesh over-garments reduced the tsetse biting rate by 75%. (See above study).