Flood turning brook periodically into a river (7)
I believe the answer is:
torrent
'flood' is the definition.
(a torrent is a large amount of water)
'turning brook periodically into a river' is the wordplay.
'turning' shows that the letters should be reversed in order.
'periodically' means one should take alternating letters (letters must be selected 'every so often').
'into' means one lot of letters goes inside another.
'a river' becomes 'trent' (River Trent).
The alternating letters of 'brook' are 'ro'.
'ro' written backwards gives 'or'.
'or' going inside 'trent' is 'TORRENT'.
(Other definitions for torrent that I've seen before include "Swift or violent stream" , "Heavy rain" , "Strong stream" , "Violently fast stream of water" , "A rushing stream of water" .)