Knight I sent up for the flag (4)
I believe the answer is:
iris
'the flag' is the definition.
('iris' can be a synonym of 'flag')
'knight i sent up' is the wordplay.
'knight' becomes 'sir' (I've seen this before).
'sent up' is a reversal indicator.
'sir'+'i'='siri'
'siri' in reverse letter order is 'IRIS'.
'for' is the link.
(Other definitions for iris that I've seen before include "In the middle of which pupil is opening" , "Camera diaphragm" , "Flower and eye membrane" , "Flower - rainbow" , "Greek goddess of the rainbow" .)