Seamen initially lower boat (4)
I believe the answer is:
scow
'boat' is the definition.
(I know that scow is a type of boat)
'seamen initially lower' is the wordplay.
'initially' indicates taking the first letters.
'lower' becomes 'cow' (something which lows - makes cow sounds).
The initial letter of 'seamen' is 's'.
's'+'cow'='SCOW'
(Other definitions for scow that I've seen before include "Vessel" , "Wide-beamed dinghy" , "Maybe lighter" , "Low boat" , "Unpowered freight barge" .)