Trainee misplaced the last of finder's fee (8)
I believe the answer is:
retainer
'fee' is the definition.
(retainer is a kind of fee)
'trainee misplaced the last of finder's' is the wordplay.
'misplaced' indicates an anagram (letters in the wrong places).
'the last of' indicates one should take the final letters.
The final letter of 'finder' is 'r'.
'trainee' is an anagram of 'retaine'.
'retaine'+'r'='RETAINER'
(Other definitions for retainer that I've seen before include "given payment in advance" , "Fee paid to book the services of someone" , "Down-payment for Eritrean" , "Fee; long-term servant" , "Family servant of long standing - fee charged in advance" .)