Treat badly tropical fish (5)
I believe the answer is:
tetra
'tropical fish' is the definition.
(I've seen this before)
'treat badly' is the wordplay.
'badly' is an anagram indicator.
'treat' is an anagram of 'TETRA'.
(Other definitions for tetra that I've seen before include "Bright fish; four (prefix)" , "Wrongful act (Law)" , "rarely seen flower" , "in groups of four" , "starter for four" .)