Callously done when covered by reptilian gore (2,4,5)
I believe the answer is:
in cold blood
'callously' is the definition.
Both the answer and definition are adverbs. Maybe you can see an association between them that I don't see?
'done when covered by reptilian gore' is the wordplay.
I cannot really see how this works, but
'covered' could be 'in' (I've seen this in other clues) and 'in' is found in the answer.
'gore' could be 'blood' (synonyms) and 'blood' is found within the answer.
The remaining letters 'cold' is a valid word which might be clued in a way I don't understand.
This explanation may well be incorrect...
Can you help me to learn more?
(Other definitions for in cold blood that I've seen before include "with evil intent" , "Acting without mercy" , "Ruthlessly" , "Without feeling or mercy" , "snakes operate?" .)