Take on aggregate during sea trip (6)
I believe the answer is:
assume
'take on' is the definition.
(I know that take on can be written as assume)
'aggregate during sea trip' is the wordplay.
'aggregate' becomes 'sum' (synonyms).
'during' is an insertion indicator.
'trip' indicates anagramming the letters.
'sea' with letters rearranged gives 'ase'.
'sum' put within 'ase' is 'ASSUME'.
(Other definitions for assume that I've seen before include "Take for granted or take upon oneself" , "Accept as true without proof" , "Take as true without proof" , "Suppose - appropriate - feign - adopt" , "Guess" .)