Back in the den Terry finishes off the second test (7)
I believe the answer is:
retrial
'the second test' is the definition.
(a second trial or test)
'back in the den terry finishes' is the wordplay.
'back in' becomes 'retr' (I've seen this in another clue).
'the den terry' becomes 'ial' (I can't explain this - if you can you should believe this answer much more).
'finishes' means one lot of letters go next to another (the solver is told which letters finish the word).
'retr'+'ial'='RETRIAL'
'off' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for retrial that I've seen before include "Hearing repeat" , "Further action" , "Second court hearing" , "Revived case" , "test, after failure in first" .)