Gentle escape, but not initially (5)
I believe the answer is:
light
'gentle' is the definition.
(synonyms)
'escape but not initially' is the wordplay.
'escape' becomes 'flight' (synonyms).
'but not initially' means to remove the first letter.
'flight' with its first letter removed is 'LIGHT'.
(Other definitions for light that I've seen before include "Portable" , "Visible rays" , "See 19" , "Easy to carry" , "Low in density" .)