They print letters on electoral system (7)
I believe the answer is:
presses
'they print' is the definition.
Although both the answer and 'print' are verbs, they are not in the same form of the verb.
'letters on electoral system' is the wordplay.
'letters' becomes 'esses' (more than one S).
'on' is a charade indicator (letters next to each other).
'electoral system' becomes 'pr' (proportional representation).
'esses' put after 'pr' is 'PRESSES'.
Can you help me to learn more?
(Other definitions for presses that I've seen before include "Machines for printing" , "Pushes" , "Machines to extract, eg, olive oil" , "Irons" .)