Bull gets cheek of a primula (5)
I believe the answer is:
oxlip
'primula' is the definition.
(oxlip is a kind of primula)
'bull gets cheek' is the wordplay.
'bull' becomes 'ox'.
'gets' means one lot of letters go next to another.
'cheek' becomes 'lip' (both can mean insolence or rudeness).
'ox'+'lip'='OXLIP'
'of a' is the link.
(Other definitions for oxlip that I've seen before include "Flowery name" , "It's a bloomer" , "Type of primula" , "Woodland plant" , "Flowering plant" .)