Rushes small deer back (5)
I believe the answer is:
reeds
'rushes' is the definition.
(I've seen this in another clue)
'small deer back' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'back' says the letters should be written in reverse.
's'+'deer'='sdeer'
'sdeer' reversed gives 'REEDS'.
(Other definitions for reeds that I've seen before include "Tall riverside grasses" , "Water plants" , "Woodwind instruments" , "Marsh plants with tall straight stems" , "River grasses" .)