Chief person holding the line (4)
I believe the answer is:
main
'chief' is the definition.
(I know that chief can be written as main)
'person holding the line' is the wordplay.
'person' becomes 'man' (man is a kind of person).
'holding' becomes 'i' (I can't explain this - if you can you should give a lot more credence to this answer).
'the line' means one lot of letters goes inside another.
'man' enclosing 'i' is 'MAIN'.
Can you help me to learn more?
(Other definitions for main that I've seen before include "associated with the Spanish?" , "Chief - principal electricity cable" , "Dominant" , "Principal motivation" , "Cardinal" .)