In putting the last tenders out, they do the wrong thing (9)
I believe the answer is:
offenders
'tenders out they do the wrong thing' is the definition.
The definition suggests an adverb but the answer is not.
'in putting the last' is the wordplay.
'in' means one lot of letters goes inside another.
'putting' becomes 'offers' (I can't justify this - if you can you should give a lot more credence to this answer).
'the last' becomes 'end' (I've seen this in another clue).
'offers' going around 'end' is 'OFFENDERS'.
Can you help me to learn more?
(Other definitions for offenders that I've seen before include "Perhaps thieves" , "Wrongdoers" , "Miscreants" , "They break the law" , "Criminals" .)