He unexpectedly helped a parent aboard cargo train in motion (4,9)
I believe the answer is:
good samaritan
'he unexpectedly helped' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'a parent aboard cargo train in motion' is the wordplay.
'parent' becomes 'ma' (ma can mean mum or mother).
'aboard' means one lot of letters goes inside another.
'cargo' becomes 'goods' (I can't explain this - if you can you should believe this answer much more).
'in motion' indicates anagramming the letters (I've seen this in other clues).
'a'+'ma'='ama'
'train' is an anagram of 'ritan'.
'goods'+'ritan'='goodsritan'
'ama' put into 'goodsritan' is 'GOOD SAMARITAN'.
Can you help me to learn more?
(Other definitions for good samaritan that I've seen before include "helping hand is required" , "One giving disinterested help, maybe to a stranger" , "One kindly rescuing" , "helpful type" , "altruist" .)