Birds turned up on roof of reprocessing plant (8)
I believe the answer is:
larkspur
'plant' is the definition.
(larkspur is a kind of plant)
'birds turned up on roof of reprocessing' is the wordplay.
'birds' becomes 'larks' (lark is a kind of bird).
'turned' says the letters should be written in reverse.
'up on' becomes 'up' (similar in meaning).
'roof of' suggests taking the first letters (in a down clue, the roof is the top letter).
The first letter of 'reprocessing' is 'r'.
'up' reversed gives 'pu'.
'larks'+'pu'+'r'='LARKSPUR'
(Other definitions for larkspur that I've seen before include "it grows" , "Garden flower of delphinium family" , "Delphinium-like flower" , "Mediterranean plant of the buttercup family" , "Delphinium-like plant" .)