Likeness I am to have a long time (5)
I believe the answer is:
image
'likeness' is the definition.
(I've seen this in another clue)
'i am to have a long time' is the wordplay.
'i am' becomes 'I'm' (contraction).
'to have' means one lot of letters go next to another.
'a long time' becomes 'age' (an age is a period of time).
'im'+'age'='IMAGE'
(Other definitions for image that I've seen before include "Likeness; metaphor" , "Iconic representation" , "Visualisation" , "Visible impression" , "Photo or likeness" .)