Expletive coming from boathouse (4)
I believe the answer is:
oath
'expletive' is the definition.
('oath' can be a synonym of 'expletive')
'coming from boathouse' is the wordplay.
'coming from' indicates a hidden word.
'OATH' is hidden within 'boathouse'.
(Other definitions for oath that I've seen before include "Courtroom promise" , "One sworn" , "A curse or a vow" , "blasphemy" , "A word with a witness" .)