Watercourse constructed initially from the bottom (5)
I believe the answer is:
canal
'watercourse' is the definition.
(I know that canal is a type of watercourse)
'constructed initially from the bottom' is the wordplay.
'initially' indicates taking the first letters.
'from the bottom' becomes 'anal' (relating to the anus).
The initial letter of 'constructed' is 'c'.
'c'+'anal'='CANAL'
(Other definitions for canal that I've seen before include "Inland waterway" , "Long, narrow strip of water" , "Man-made waterway" , "Grand Union perhaps" , "Passage" .)