Get ready to slim down after purge, oddly (7)
I believe the answer is:
prepare
'get ready to' is the definition.
(thesaurus)
'slim down after purge oddly' is the wordplay.
'slim down' becomes 'pare' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'after' says to put letters next to each other.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'purge' are 'pre'.
'pare' put after 'pre' is 'PREPARE'.
Can you help me to learn more?
(Other definitions for prepare that I've seen before include "[GET READY] (3 of 3)" , "Make ready or suitable" , "Fit out" , "study" , "Make (something) ready" .)