Lie about insect being in food that's sent back (8)
I believe the answer is:
stagnate
'lie about' is the definition.
'insect being in food that's sent back' is the wordplay.
'insect' becomes 'gnat' (gnat is a kind of insect**).
'being in' indicates putting letters inside.
'food' becomes 'eats' (eats is a kind of food).
'that's sent back' is a reversal indicator.
'eats' in reverse letter order is 'stae'.
'gnat' going inside 'stae' is 'STAGNATE'.
(Other definitions for stagnate that I've seen before include "Become inactive (and malodorous)" , "Become still or stale through inactivity" , "resemble pond water?" , "Become foul from inertia" , "Become stale or inactive" .)