Supporter's run on board plane? (4)
I believe the answer is:
tree
'plane?' is the definition.
(I've seen this before)
'supporter's run on board' is the wordplay.
'supporter' becomes 'tee' (I've seen this in other clues).
'run' becomes 'r' (cricket abbreviation).
'on board' indicates putting letters inside.
'tee' placed around 'r' is 'TREE'.
(Other definitions for tree that I've seen before include "Ash, chestnut, or araucaria, for example" , "Chestnut or elm?" , "Diagram representing different relationships" , "Forest plant" , "Tall wooden plant" .)