Well,one of the earliest of her novels is a Poirot,
THE MYSTERIOUS AFFAIR AT STYLES
Written in 1920.
However although this has a male suspect(and murderer) Alfred Inglethorp,he does have a female accomplice,who is Evelyn Howard;so this may exclude it for you.
For a solitary male suspect we have to turn to THE MURDER OF ROGER ACKROYD
written in 1926 where the suspect/murdere is Dr James Sheppard,who by the way is a close friend of Poirot!
This is the novel that got Christie into trouble,because the murdere is also the narrator,and at that time to have such a plot device was considered underhand as it deceived the reader.
If course after she had done it,many other writers used the same device.
None of the short stories,not the Miss MArple books (written some time later than the Poirot's) seem to have male suspects etc before 1920(or 1926,if you want a sole male).
Have a look at this excellent Christie site:~
http://www.gbtamc.co.uk/et/index.php?id=674