Plant small flower over heather (8)
I believe the answer is:
seedling
'plant' is the definition.
(seedling is a kind of plant)
'small flower over heather' is the wordplay.
'small' becomes 's' (abbreviation - e.g. clothes size).
'flower' becomes 'dee' (River Dee).
'over' says the letters should be written in reverse.
'heather' becomes 'ling' (synonyms).
'dee' backwards is 'eed'.
's'+'eed'+'ling'='SEEDLING'
(Other definitions for seedling that I've seen before include "Young plant" , "Tiny plant" , "Small plant" .)