One insect, or another with small mark on, let free (4,6)
I believe the answer is:
stag beetle
'one insect' is the definition.
(stag beetle is a kind of insect)
'another with small mark on let free' is the wordplay.
I cannot really understand how this works, but
'another' could be 'e' (I've seen this in other clues) and 'e' is located in the answer.
'small' could be 's' (abbreviation - e.g. clothes size) and 's' is found within the answer.
'mark' could be 'tag' (synonyms) and 'tag' is found within the answer.
an anagram of 'let' is 'etl' which is located in the answer.
The remaining letters 'be' is a valid word which might be clued in a way I don't see.
This may be the basis of the clue (or it may be nonsense).
'or' acts as a link.
(Other definitions for stag beetle that I've seen before include "Insect with antler-like jaws" , "Horny little creature" , "Six-footer" , "Large insect with branched mandibles" , "something some folk find creepy" .)