Severe if back on board (5)
I believe the answer is:
stern
This might be a double definition.
'severe' is the first definition.
(I know that severe can be written as stern)
'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' is the link.
(Other definitions for stern that I've seen before include "Strict, austere" , "Looking severe" , "Unpleasantly serious" , "Severe part of the ship?" , "Old star" .)