He's done the wrong thing in accepting the last tender (8)
I believe the answer is:
offender
'he's done the wrong thing' is the definition.
The answer is a person as well as being a singular noun. This is suggested by the definition.
'accepting the last tender' is the wordplay.
'accepting' is an insertion indicator (inserted letters are accepted).
'the last' becomes 'end' (I've seen this in another clue).
'tender' becomes 'offer' (synonyms).
'end' inserted within 'offer' is 'OFFENDER'.
'in' is the link.
Can you help me to learn more?
(Other definitions for offender that I've seen before include "Criminal" , "One breaking rule" , "Law breaker" , "miscreant" , "guilty party" .)