Principal fellow getting out of line (4)
I believe the answer is:
main
'principal' is the definition.
(both can mean major)
'fellow getting out of line' is the wordplay.
'fellow' becomes 'man' (fellow can mean a man).
'getting' indicates putting letters inside.
'out of line' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'man' enclosing 'i' is 'MAIN'.
Can you help me to learn more?
(Other definitions for main that I've seen before include "Chief; sea; pipe" , "location for 9 [ARMADA]" , "Most important - high seas?" , "Chief, primary" , "Chief in importance" .)