Old English flower (3)
I believe the answer is:
exe
'flower' is the definition.
(River Exe)
'old english' is the wordplay.
'old' becomes 'ex'.
'english' becomes 'e' (abbreviation).
'ex'+'e'='EXE'
(Other definitions for exe that I've seen before include "River of SW England" , "Devon river" , "Tiverton's river" , "Topsham's river" , "West Country river" .)