The chap holding the line is the chief (4)
I believe the answer is:
main
'the chief' is the definition.
(I've seen this before)
'the chap holding the line' is the wordplay.
'the chap' becomes 'Man' (chap can mean a man).
'holding' indicates putting letters inside.
'the line' becomes 'i' (I can't justify this - if you can you should give a lot more credence to this answer).
'man' going around 'i' is 'MAIN'.
'is' is the link.
Can you help me to learn more?
(Other definitions for main that I've seen before include "Salt water" , "Principal motivation" , "Ocean or principal pipe" , "Most important element" , "Foremost" .)