.. bird seen on board? (4)
I believe the answer is:
rook
'bird seen on board?' is the definition.
'rook' can be an answer for 'bird' (I've seen this before). I am not sure about the 'seen on board?' bit.
'bird seen on' is the wordplay.
'bird' becomes 'oo'.
'seen' becomes 'rk' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'on' is an insertion indicator (as in clothing 'on' a person).
'oo' placed inside 'rk' is 'ROOK'.
(Other definitions for rook that I've seen before include "Black bird - cheat - chess piece" , "Do < cornerpiece" , "16" , "Gregarious crow" , "Chess piece or crow" .)