New line for a trooper (8)

I believe the answer is:
operator
'new line' is the definition.
Although both the answer and definition are singular nouns, I can't see how one could define the other.
'a trooper' is the wordplay.
'a trooper' anagrams to 'OPERATOR'.
However, I'm not sure how the anagram is indicated.
'for' acts as a link.
Can you help me to learn more?
(Other definitions for operator that I've seen before include "Mathematical symbol" , "Manipulator" , "Person who directs or controls, machine say (8)." , "Agent that runs some apparatus or machine" , "One using an item of equipment" .)
