I'm standing up twice before Concorde's first take off (5)
I believe the answer is:
mimic
'take off' is the definition.
(I've seen this before)
'i'm standing up twice before concorde's first' is the wordplay.
'standing up' says the letters should be written in reverse (in a down clue up is reverse order).
'twice' shows that letters should be duplicated.
'before' means one lot of letters go next to another.
'concorde's first' becomes 'c' (1st letter of 'concorde').
'im' in reverse letter order is 'mi'.
'mi' doubled is 'mimi'.
'mimi'+'c'='MIMIC'
(Other definitions for mimic that I've seen before include "Impressionist" , "Imitate in order to ridicule" , "Imitate satirically" , "Go round a bend (in car)" , "Imitate, especially for satirical effect" .)