One's to bring in Conservative by southern entrance (6)
I believe the answer is:
access
'entrance' is the definition.
(I've seen this before)
'one's to bring in conservative by southern' is the wordplay.
'one's' becomes 'aces' (ace has a value of one in some card games).
'to bring in' means one lot of letters goes inside another.
'conservative' becomes 'C' (UK political abbreviation).
'by' means one lot of letters go next to another.
'southern' becomes 's' (abbreviation).
'aces' enclosing 'c' is 'acces'.
'acces'+'s'='ACCESS'
(Other definitions for access that I've seen before include "drive, perhaps" , "Admission" , "Credit" , "The right to use something or see someone" , "outburst" .)