Ship's captain (7)
(Other definitions for number one that I've seen before include "best record" , "top-selling record" , "Top-of-the-charts hit" , "it concerns egoists" , "officer on board?" .)
'ship's captain' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for skipper that I've seen before include "Chap aboard trawler" , "Captain - small butterfly - fish" , "One who misses" , "Trawler boss" , "Head" .)