Yon Conservative, the female PM once (8)
I believe the answer is:
thatcher
'pm once' is the definition.
(I know that Thatcher was a Prime Minister of the United Kingdom)
'yon conservative the female' is the wordplay.
'yon' becomes 'that' (I've seen this before).
'conservative' becomes 'C' (UK political abbreviation).
'the female' becomes 'her' (I've seen this in another clue).
'that'+'c'+'her'='THATCHER'
(Other definitions for thatcher that I've seen before include "who created Boris's hairstyle?" , "Baroness --, former PM" , "Worker on ladder" , "Stateswoman" , "Iron 17 across" .)