Severe if back on board (5)
I believe the answer is:
stern
This clue could be a double definition.
'severe' is the first definition.
(I've seen this before)
'back on board' is the second definition.
'stern' can be an answer for 'back' (stern means the back of a boat). I am not sure about the 'on board' bit.
'if' acts as a link.
(Other definitions for stern that I've seen before include "Strict, austere" , "Looking severe" , "Unpleasantly serious" , "Severe part of the ship?" , "Old star" .)