Band trained to go in to drop flood defence (7)
I believe the answer is:
sandbag
'flood defence' is the definition.
(I've seen this before)
'band trained to go in to drop' is the wordplay.
'trained' indicates anagramming the letters (to train can mean to grow something into a shape).
'to go in' means one lot of letters goes inside another.
'to drop' becomes 'sag' (sagging is a kind of dropping**).
'band' is an anagram of 'ndba'.
'ndba' inserted into 'sag' is 'SANDBAG'.
(Other definitions for sandbag that I've seen before include "defensive article" , "It's used to protect against flooding" , "heavy weight" , "Item used in floods, or to hit someone with" , "Defence against flooding" .)