Clustering plant associated with Stirling, nominally (4)
I believe the answer is:
moss
This clue is a double definition.
'clustering plant' is the first definition.
'associated with stirling nominally' is the second definition.
(Stirling Moss was a racing driver)
(Other definitions for moss that I've seen before include "Plant growing on many lawns" , "Type of plant growing in damp places" , "It carpets moist shady places" , "Parasitic plant" , "Plant forming most of my lawn!" .)