Plane maybe rising in sheer trajectory (4)
I believe the answer is:
tree
'plane maybe' is the definition.
(plane is a type of tree)
'rising in sheer trajectory' is the wordplay.
'rising' says the letters should be written backwards (going up in a down clue).
'in' says the answer is hidden in the clue.
'eert' can be found hidden inside 'sheer trajectory'.
'eert' reversed gives 'TREE'.
(Other definitions for tree that I've seen before include "Plane perhaps" , "Sequoia, for example" , "Eg, alder" , "eg Box" , "famous actor" .)