Old for new exchange was badly trained (5-2)
I believe the answer is:
trade-in
'old for new exchange' is the definition.
I can't judge whether this defines the answer.
'badly trained' is the wordplay.
'badly' is an anagram indicator.
'trained' is an anagram of 'TRADE-IN'.
'was' is the link.
Can you help me to learn more?
(Other definitions for trade-in that I've seen before include "Older article used in part payment to retailer" , "I daren't (anag)" , "Part-exchanged item" , "Article used in part-exchange" , "Old item given in part payment for a new one" .)