Rook in bush, possibly hydrangea (5)
I believe the answer is:
shrub
'hydrangea' is the definition.
(thesaurus)
'rook in bush possibly' is the wordplay.
'rook' becomes 'R' (chess abbreviation).
'in' is an insertion indicator.
'possibly' is an anagram indicator.
'bush' with letters rearranged gives 'shub'.
'r' inserted within 'shub' is 'SHRUB'.
(Other definitions for shrub that I've seen before include "box, say" , "Bushy plant" , "Woody plant, bush" , "Woody plant smaller than a tree" , "Perhaps Heather" .)