Roof of bus station rebuilt for plant expert (8)
I believe the answer is:
botanist
'plant expert' is the definition.
(I've seen this before)
'roof of bus station rebuilt' is the wordplay.
'roof of' says to take the initial letters (in a down clue, the top letter).
'rebuilt' indicates an anagram.
The first letter of 'bus' is 'b'.
'station' anagrammed gives 'otanist'.
'b'+'otanist'='BOTANIST'
'for' is the link.
(Other definitions for botanist that I've seen before include "Expert in study of plants and trees" , "Student of plants" , "Expert in plant biology" , "Plant scientist" , "Expert on plant life" .)