Submerge head and keep under flat-bottomed boat (4)
I believe the answer is:
scow
'flat-bottomed boat' is the definition.
(I've seen this before)
'submerge head and keep under' is the wordplay.
'head' says to take the initial letters.
'and' says to put letters next to each other.
'keep under' becomes 'cow' (to cow someone is to intimidate or keep them under).
The initial letter of 'submerge' is 's'.
's'+'cow'='SCOW'
(Other definitions for scow that I've seen before include "Vessel" , "Low boat" , "Maybe lighter" , "Flat-bottomed boat for cows" , "ighter" .)