Entire triangle at sea (8)
I believe the answer is:
integral
'entire' is the definition.
(I've seen this before)
'triangle at sea' is the wordplay.
'at sea' is an anagram indicator.
'triangle' with letters rearranged gives 'INTEGRAL'.
(Other definitions for integral that I've seen before include "Forming an essential part" , "Triangle/relating/altering/alerting (anag)" , "contributing to unity" , "Belonging as an essential part" , "form whole" .)