Go ahead and agree on flight taking off at first (5,5)
I believe the answer is:
green light
'go ahead' is the definition.
(I've seen this in another clue)
'and agree on flight taking off at first' is the wordplay.
'and' becomes 'n' (common abbreviation for 'and').
'agree' becomes 'gree' (I am not sure about this - if you are sure you should believe this answer much more).
'on' means one lot of letters go next to another.
'taking off at first' means to remove the first letter.
'flight' with its initial letter taken off is 'light'.
'n' put after 'gree' is 'green'.
'green'+'light'='GREEN LIGHT'
(Other definitions for green light that I've seen before include "OK" , "Seal of approval" , "Signal of consent" , "What one has to go on" , "Go-ahead" .)