Small insect eats nothing but sand (6)
I believe the answer is:
smooth
'sand' is the definition.
(I know that sand is a more specific form of the action smooth)
'small insect eats nothing' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'insect' becomes 'moth' (moth is a kind of insect).
'eats' is an insertion indicator.
'nothing' becomes 'o' (O resembles 0).
'moth' going around 'o' is 'mooth'.
's'+'mooth'='SMOOTH'
'but' acts as a link.
(Other definitions for smooth that I've seen before include "Polished" , "Still" , "without difficulty" , "Having even surface with no projections" , "Lacking roughness" .)