Flowers grew on Sunday (5)
I believe the answer is:
roses
'flowers' is the definition.
(rose is a type of flower)
'grew on sunday' is the wordplay.
'grew' becomes 'rose' (rise can mean to grow or increase).
'on' is a charade indicator (letters next to each other) (in a down clue, letters appear on others).
'sunday' becomes 's'.
'rose'+'s'='ROSES'
(Other definitions for roses that I've seen before include "Bloomers" , "Flowers, usually red, pink, white or yellow" , "Ramblers, for example" , "northern counties, symbolically" , "''. . . . . are red, violets are blue''" .)