Traditional girl supporting Conservative in charge (7)
I believe the answer is:
classic
'traditional' is the definition.
(I've seen this before)
'girl supporting conservative in charge' is the wordplay.
'girl' becomes 'lass' (lass is a kind of girl**).
'supporting' says to put letters next to each other (in a down clue, some letters go below others).
'conservative' becomes 'c' (abbreviation commonly used in UK politics).
'in charge' becomes 'ic'.
'lass' after 'c' is 'class'.
'class'+'ic'='CLASSIC'
(Other definitions for classic that I've seen before include "Serving as a standard - of the highest quality" , "Work of established excellence (7)" , "work of the highest quality" , "Important horse race" , "Derby, say" .)