Cleg is the north country term for horseflies, deriving from ON kleggi (cf. Mod. Norwegian klegg). Horseflies are ugly, vicious ,bloated, biting insects which suck the blood from exposed parts of fishermen, usually in Scotland or Ireland. They are impossible to deter with insect repellent; insect repellent attracts them. They should be smacked dead before they bite, if possible. Cleg-smacking is an alternative pursuit to fishing during long, hot, sweaty and windless days when the fish are as sulky as the weather (McCully, 1993).

Ref - McCully, C.B. Flyfishing: A book of words, Oxford, Oxford university press, 1993.