Beast of burden, one oddly biting head of harmless reptile (9)
I believe the answer is:
Here is my best explanation:
'reptile' is the definition.
(chameleon is a kind of reptile)
'beast of burden one oddly biting head of harmless' is the subsidiary indication.
'beast of burden' becomes 'camel' (I've seen this before**).
'oddly' indicates an anagram.
'biting' is an insertion indicator (I've seen this in other clues).
'head of' suggests taking the first letters.
The initial letter of 'harmless' is 'h'.
'one' with letters rearranged gives 'eon'.
'cameleon' going around 'h' is 'chameleon'.
(Other definitions for chameleon that I've seen before include "Someone who changes position to suit the situation", "Animal that changes colour", "Lizard whose skin changes colour", "Lizard that can adapt its colours to background", "One to blend in", "It blends", "I'll adapt to fit in", "Colour-changing lizard", "blender" and "fickle person".)