Piece of silicon Greek character found in farmer's yield by motorway (9)
I believe the answer is:
microchip
'piece of silicon' is the definition.
(microchips are usually made of silicon)
'greek character found in farmer's yield by motorway' is the wordplay.
'greek character' becomes 'chi' (letter from the Greek alphabet).
'found in' means one lot of letters goes inside another.
'farmer's yield' becomes 'crop' (I've seen this in another clue).
'by' means one lot of letters go next to another.
'motorway' becomes 'MI' (resembles M1, English motorway).
'chi' put inside 'crop' is 'crochip'.
'crochip' put after 'mi' is 'MICROCHIP'.
(Other definitions for microchip that I've seen before include "Integrated circuit" , "Tiny proportion of potato" , "little conductor" , "processor?" , "You may find it inside pet!" .)