By trapping the water, showing to be intelligent (6)
I believe the answer is:
brainy
'to be intelligent' is the definition.
('brainy' can be similar in meaning to 'intelligent')
'by trapping the water' is the wordplay.
'trapping' means one lot of letters goes inside another.
'the water' becomes 'rain' (rain is a kind of water).
'by' placed around 'rain' is 'BRAINY'.
'showing' acts as a link.
(Other definitions for brainy that I've seen before include "Highly intelligent" , "smart?" , "Showing intelligence" , "Intellectual" , "Bright" .)