Heavy machines Greek character guided over waterways (4-7)

'heavy machines' is the definition.
(pile-drivers are heavy)

'greek character guided over waterways' is the wordplay.
'greek character' becomes 'pi' (letter from the Greek alphabet).
'guided' becomes 'led' ('lead' can be a synonym of 'guide').
'over' is a charade indicator (letters next to each other) (in a down clue, some letters go over others).
'waterways' becomes 'rivers' (I've seen this in another clue).
'pi'+'led'+'rivers'='PILE-DRIVERS'