I am a long time on extremes of ready representation (7)
I believe the answer is:
imagery
'representation' is the definition.
(thesaurus)
'i am a long time on extremes of ready' is the wordplay.
'i am' becomes 'I'm' (contraction).
'a long time' becomes 'age' (an age is a period of time).
'on' is a charade indicator (letters next to each other).
'extremes of' means to remove the middle letters (only the letters at the extremes of the word).
'ready' with its middle taken out is 'ry'.
'im'+'age'+'ry'='IMAGERY'
(Other definitions for imagery that I've seen before include "illustrative effects" , "Visual symbolism" , "Language that produces pictures in readers' minds" , "I'm gayer due to mental pictures" , "Descriptive language" .)