A glossy back on a lizard (5)
I believe the answer is:
agama
'lizard' is the definition.
(agama is a kind of lizard)
'a glossy back on a' is the wordplay.
'glossy' becomes 'mag' (I've seen this in other clues).
'back' says the letters should be written in reverse.
'on' means one lot of letters go next to another.
'mag' in reverse letter order is 'gam'.
'a'+'gam'+'a'='AGAMA'
(Other definitions for agama that I've seen before include "Reptile" , "Small terrestrial Old World lizard" .)