Egghead departing in vessel (6)
I believe the answer is:
boffin
'egghead' is the definition.
'departing in vessel' is the wordplay.
I cannot quite understand how this works, but
'in' is within the answer.
The remaining letters 'boff' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for boffin that I've seen before include "Back-room boy" , "(Government) scientist" , "Research scientist" , "Scientist (in the back room?)" , "Scientist (sl.)" .)