Reversing in glider really was wrong (5)
I believe the answer is:
erred
'was wrong' is the definition.
(I've seen this in another clue)
'reversing in glider really' is the wordplay.
'reversing' shows that the letters should be reversed in order.
'in' says the answer is hidden in the clue.
'derre' is hidden within 'glider really'.
'derre' written backwards gives 'ERRED'.
(Other definitions for erred that I've seen before include "Made mistake, was incorrect" , "Was mistaken" , "Committed a sin" , "Made mistakes" , "Went astray" .)