Insect worn out eating top of gnarly plant (4,6)
I believe the answer is:
stag beetle
'insect' is the definition.
(stag beetle is a kind of insect)
'worn out eating top of gnarly plant' is the wordplay.
'worn out' becomes 'stale' (stale can mean worn out or hackneyed).
'eating' means one lot of letters goes inside another.
'top of' indicates taking the first letters.
'plant' becomes 'beet' (beet is a kind of plant).
The first letter of 'gnarly' is 'g'.
'g'+'beet'='gbeet'
'stale' placed around 'gbeet' is 'STAG-BEETLE'.
(Other definitions for stag beetle that I've seen before include "Horny little creature" , "something some folk find creepy" , "Insect with antler-like jaws" , "Large insect with branched mandibles" , "Six-footer" .)