Rook in bush, possibly forsythia? (5)
I believe the answer is:
shrub
'forsythia?' is the definition.
(forsythia is a kind of shrub)
'rook in bush possibly' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'in' indicates putting letters inside.
'possibly' is an anagram indicator.
'bush' with letters rearranged gives 'shub'.
'r' inserted inside 'shub' is 'SHRUB'.
(Other definitions for shrub that I've seen before include "Small woody plant" , "Brush? (anagr.)" , "box, say" , "Bushy plant" , "Perhaps Heather" .)