Regular light infantry starts out with one who doesn't belong (9)
I believe the answer is:
guerrilla
'one who doesn't belong' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'regular light infantry starts out' is the wordplay.
'starts' suggests taking the first letters.
'out' indicates an anagram (out can mean wrong or inaccurate).
The initial letters of 'light infantry' is 'li'.
'regular'+'li'='regularli'
'regularli' with letters rearranged gives 'GUERRILLA'.
'with' acts as a link.
Can you help me to learn more?
(Other definitions for guerrilla that I've seen before include "Person who fights as one of small independent group" , "Irregular fighter" , "Underground fighter" , "in conflict to secure political end?" , "Such tactics" .)