Counsel, truthfully, brought up caged reptiles (7)
I believe the answer is:
turtles
'reptiles' is the definition.
(I know this)
'counsel truthfully brought up caged' is the wordplay.
'caged' indicates a hidden word.
'brought up' says the letters should be written backwards (in a down clue, letters go up).
'seltrut' is hidden within 'counsel truthfully'.
'seltrut' in reverse letter order is 'TURTLES'.
(Other definitions for turtles that I've seen before include "Slow swimmers" , "Let rust settle on sea-creatures" , "Marine reptiles" , "The reptiles are let rust" , "Reptiles with bony shell and flipper-like limbs" .)