Terrible chap lifted up reptile for the main explorer (9)
I believe the answer is:
navigator
'the main explorer' is the definition.
(an explorer - 'the main' can mean the sea)
'terrible chap lifted up reptile' is the wordplay.
'terrible chap' becomes 'Ivan' (Ivan the Terrible - Ivan IV of Russia).
'lifted up' says the letters should be written backwards (in a down clue, letters go up).
'reptile' becomes 'gator' (short for alligator).
'ivan' back-to-front is 'navi'.
'navi'+'gator'='NAVIGATOR'
'for' acts as a link.
(Other definitions for navigator that I've seen before include "Co-driver" , "Assistant to driver" , "Person steering course" , "Pilot" , "crew member" .)