Exclusive lot is terribly exclusive initially (5)
I believe the answer is:
elite
'exclusive' is the definition.
(I've seen this before)
'lot is terribly exclusive initially' is the wordplay.
'lot' becomes 'el' (I can't explain this - if you can you should give a lot more credence to this answer).
'initially' indicates taking the first letters.
The initial letters of 'is terribly exclusive' is 'ite'.
'el'+'ite'='ELITE'
Can you help me to learn more?
(Other definitions for elite that I've seen before include "Cream, the pick" , "A privileged or superior social group" , "Type size; superior group" , "Selected as the best" , "Choice; best" .)