I'm a long time getting the picture (5)
I believe the answer is:
image
'the picture' is the definition.
('image' can be a synonym of 'picture')
'i'm a long time getting' is the wordplay.
'a long time' becomes 'age' ('age' can be a synonym of 'long time').
'getting' means one lot of letters go next to another.
'im'+'age'='IMAGE'
(Other definitions for image that I've seen before include "Vision" , "Mental or physical representation" , "sculpture" , "Mental picture; idol" , "External impression" .)