Old PM with monarch and girl (7)
I believe the answer is:
heather
'girl' is the definition.
'old pm with monarch' is the wordplay.
'old pm' becomes 'Heath' (prime minister Edward Heath).
'with' says to put letters next to each other.
'monarch' becomes 'ER' (abbreviation for Elizabeth Regina, British Queen).
'heath'+'er'='HEATHER'
'and' is the link.
(Other definitions for heather that I've seen before include "Low shrub; mixed-colour (yarn)" , "Girl" , "Another term for ling" , "Girl's name - ling" , "thing seen on the moors" .)