For Poirot, some trouble following each villain (9)
I believe the answer is:
desperado
'villain' is the definition.
The answer and definition can be both people as well as being singular nouns.
Maybe they are linked in a way I don't understand?
'for poirot some trouble following each' is the wordplay.
I cannot quite understand how this works, but
'some' could be 'a' and 'a' is present in the answer.
'each' could be 'per' and 'per' is found within the answer.
This explanation may well be incorrect...
(Other definitions for desperado that I've seen before include "Bandit, outlaw" , "Reckless criminal" , "Reckless person" , "Road speed (anag.)" , "reckless type" .)