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' means one lot of letters goes inside another.
'the last' becomes 'end' (I've seen this in another clue).
'tender' becomes 'offer' (synonyms).
'end' going inside 'offer' is 'OFFENDER'.
'in' acts as a 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" .)