Initially easy to walk up over small flight (6)
I believe the answer is:
escape
'flight' is the definition.
('escape' can be a synonym of 'flight')
'initially easy to walk up over small' is the wordplay.
'initially' says to take the initial letters.
'to walk' becomes 'pace' (pacing is a kind of walking**).
'up' is a reversal indicator (in down clue: letters go upwards).
'over' means one lot of letters goes inside another.
'small' becomes 's' (abbreviation - e.g. clothes size).
The initial letter of 'easy' is 'e'.
'e'+'pace'='epace'
'epace' back-to-front is 'ecape'.
'ecape' placed around 's' is 'ESCAPE'.
(Other definitions for escape that I've seen before include "Get away from captivity" , "Elude; leakage" , "Key < flight" , "Get out of captivity, go over the wall" , "Gain freedom" .)