Principal and head meet somewhere onboard ship (7)
I believe the answer is:
maintop
'somewhere onboard ship' is the definition.
'principal and head' is the wordplay.
'principal' becomes 'main' (I've seen this before).
'and' means one lot of letters go next to another.
'head' becomes 'top' ('top' can be a synonym of 'head').
'main'+'top'='MAINTOP'
'meet' is the link.
(Other definitions for maintop that I've seen before include "Sail" , "Platform on a mast of a square-rigger - I'm on tap (anag)" , "Superior position on board" , "Part of ship" , "one such at sea" .)