Beast of burden, one oddly biting head of harmless reptile (9)
I believe the answer is:
chameleon
'reptile' is the definition.
(chameleon is a kind of reptile)
'beast of burden one oddly biting head of harmless' is the wordplay.
'beast of burden' becomes 'camel' (I've seen this before**).
'oddly' is an anagram indicator.
'biting' indicates putting letters inside (I've seen this in other clues).
'head of' suggests taking the first letters.
The initial letter of 'harmless' is 'h'.
'one' anagrammed gives 'eon'.
'camel'+'eon'='cameleon'
'cameleon' placed 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" , "Variable-colour reptile" .)