Erica, a PM in 1970s, with monarch then (7)
I believe the answer is:
heather
'erica' is the definition.
(plant genus including heather)
'a pm in 1970s with monarch then' is the wordplay.
I cannot quite understand how this works, but
'a' is within the answer.
'monarch' could be 'er' (abbreviation for Elizabeth Regina, British Queen) and 'er' is found in the answer.
This may be the basis of the clue (or it may be nonsense).
Can you help me to learn more?
(Other definitions for heather that I've seen before include "Plant also called ling" , "Erica!" , "Girl" , "Calluna vulgaris" , "Small purple evergreen plant growing on hills" .)