Helpful chap, American uncle, arranging a train (9)
I believe the answer is:
samaritan
'helpful chap' is the definition.
(I've seen this before)
'american uncle arranging a train' is the wordplay.
'american uncle' becomes 'Sam' (Uncle Sam is a nickname for the USA).
'arranging' indicates an anagram.
'a'+'train'='atrain'
'atrain' with letters rearranged gives 'aritan'.
'sam'+'aritan'='SAMARITAN'
(Other definitions for samaritan that I've seen before include "Charitable type" , "(Good) helpful person" , "good-hearted man" , "Helpful type" , "One who helps" .)