Trouble envelops voting system for period (5)
I believe the answer is:
april
'period' is the definition.
(period of time)
'trouble envelops voting system' is the wordplay.
'trouble' becomes 'ail' (synonyms).
'envelops' means one lot of letters goes inside another (I've seen this in other clues).
'voting system' becomes 'PR' (proportional representation).
'ail' going around 'pr' is 'APRIL'.
'for' acts as a link.
(Other definitions for april that I've seen before include "Start of the financial year" , "'All in an ... evening' (Katharine Tynan)" , "Calendar month" , "on her first kid" , "''All in an ... evening / ... airs were abroad'' (by Katherine Tynan)" .)