Swear-word used in boathouse (4)
I believe the answer is:
oath
'swear-word' is the definition.
(I've seen this before)
'in boathouse' is the wordplay.
'in' says the answer is hidden in the clue.
'OATH' is hidden in the letters of 'boathouse'.
'used' is the link.
(Other definitions for oath that I've seen before include "It's taken in court" , "A solemn undertaking to tell the truth" , "Swearword or solemn word" , "in court, we take it" , "gadzooks!" .)