Head off man on board, getting better drink (8)
I believe the answer is:
nightcap
'drink' is the definition.
(I know that nightcap is a type of drink)
'head off man on board getting better' is the wordplay.
'head off' means to remove the first letter.
'man on board' becomes 'knight' (I've seen this before).
'getting better' becomes 'cap' (I can't justify this - if you can you should give a lot more credence to this answer).
'knight' with its initial letter taken away is 'night'.
'night'+'cap'='NIGHTCAP'
Can you help me to learn more?
(Other definitions for nightcap that I've seen before include "Bed wear - late drink" , "Alcoholic drink taken at bedtime" , "A drink before bed, probably alcoholic" , "Patching (anag.)" , "something you get at the eleventh hour?" .)