Speaks at length about trapping rain for the vegetables (7)
I believe the answer is:
sprouts
'the vegetables' is the definition.
(sprout is a kind of vegetable)
'speaks at length about trapping rain' is the wordplay.
I cannot quite see how this works, but
'length' could be 'r' (r is a kind of length) and 'r' is present in the answer.
'speaks' could be 'spouts' (spouting is a kind of speaking) and 'spouts' is found within the leftover letters.
This accounts for all the letters.
This explanation may well be incorrect...
'for' acts as a link.
Can you help me to learn more?
(Other definitions for sprouts that I've seen before include "Small cabbage-like veg." , "Green vegetables" , "Brussels vegetables" , "Grows vegetables" , "Green vegetable" .)