Sunday Times Cryptic 4631

[Khinjan]

Cardinal holds rare flower (6)

I think it's SEVERN but the parse escapes me. R is not an abbreviation for RARE. Am I barking up the wrong tree?

[dannyk13]

Setters often use the initial letter of a word so Severn looks ok.

[mallyh]

seven is a cardinal number + r = rare

[bookbinder]

seven is a cardinal number. Thus, seven holds rare = seve r n = River Severn.