In the film adaptation of The A.B.C. Murders in Series 2 of ITV's Agatha Christie's Poirot, the part of Franklin Clarke is played by Donald Douglas. He is slightly older than as portrayed in the original story but otherwise the portrayal is fairly faithful.
In the anime show ABC Murders, NHK's adaptation of The A.B.C. Murders, the character of Franklin Clarke is voiced by Yasunori Matsumoto. He is depicted as considerably younger than in the original, probably in his twenties--this made more plausible his ability to chat up Betty Barnard.
In the MGM adaptation The Alphabet Murders (1965), the character of Franklin Clarke is played by James Villiers. Here he is Sir Carmichael's step brother and not his brother. In this adaptation, his role is more complex. More details may be found here.