Pity old queen having to face cross physicist (10)
I believe the answer is:
rutherford
'physicist' is the definition.
(Ernest Rutherford 1871-1937 nuclear physicist)
'pity old queen having to face cross' is the wordplay.
'pity old' becomes 'ruth' (historical term for pity).
'queen' becomes 'er' (abbreviation for Elizabeth Regina).
'having to face' says to put letters next to each other (to face can mean to meet or go before).
'cross' becomes 'ford' (fording is a kind of crossing).
'ruth'+'er'+'ford'='RUTHERFORD'
(Other definitions for rutherford that I've seen before include "Actress" , "Den" , "Scientist" .)