Deep back in flashy plant (9)
I believe the answer is:
speedwell
'plant' is the definition.
(speedwell is a kind of plant)
'deep back in flashy' is the wordplay.
'back' shows that the letters should be reversed in order.
'in' indicates putting letters inside.
'flashy' becomes 'swell' (both can mean fancy or impressive).
'deep' in reverse letter order is 'peed'.
'peed' going into 'swell' is 'SPEEDWELL'.
(Other definitions for speedwell that I've seen before include "Small creeping herbaceous plant with blue or pink flowers" , "Small blue flower of the genus veronica" , "Small plant, veronica" , "Flowering plant" , "bloomer?" .)