A fly larva burrowing into herb causes destruction (8)
I believe the answer is:
sabotage
'destruction' is the definition.
(I know that sabotage is a type of destruction)
'a fly larva burrowing into herb' is the wordplay.
'fly larva' becomes 'bot' (I've seen this in another clue).
'burrowing into' is an insertion indicator.
'herb' becomes 'sage' (sage is a kind of herb).
'a'+'bot'='abot'
'abot' put inside 'sage' is 'SABOTAGE'.
'causes' acts as a link.
(Other definitions for sabotage that I've seen before include "Deliberately destroy" , "Deliberate act of destruction or disruption" , "cripple" , "Deliberate and malicious destruction" , "Deliberate wrecking" .)