1a is a thematic answer, hankir -
'rapid' is the def and it refers to the watery kind (usually pluralised as rapids) it's a 5 letter answer for the def but the wordplay gives the full entry (the other word for your 'umbrella' synonym) 'knock echoed' gives the first 3 letters of your answer followed by the usual 3 letters for 'one' with a wooden shed for the 'building' inside [the last 5 letters = the def and the first 4 letters is the thematic currency].
Sorry this is so wordy, hankir!