Tiny moving parts visible in old timepiece (4)
I believe the answer is:
sand
'parts visible in old timepiece' is the definition.
'sand' can be an answer for 'parts' (I have seen 'Parts of beach' mean 'sand' so perhaps 'parts' could also mean 'sand'). I am not sure about the 'visible in old timepiece' bit.
I don't understand how the rest of the clue works.
(Other definitions for sand that I've seen before include "Polish" , "Fine grit" , "woman writer Chopin loved" , "Seaside castle-building material" , "Granulated quartz" .)