Staff with no time to set off, being most unwell (7)
I believe the answer is:
sickest
'being most unwell' is the definition.
('sick' can be similar in meaning to 'unwell')
'staff with no time to set' is the wordplay.
I cannot really see how this works, but
'time' could be 'est' (EST is an example) and 'est' is present in the answer.
'set' could be 'sic' (synonyms) and 'sic' is located in the answer.
A single letter 'k' remains which might be clued in a way I don't see.
This explanation may well be incorrect...
'off' is the link.
Can you help me to learn more?
(Other definitions for sickest that I've seen before include "Most nauseated" , "Most poorly" , "Most ill" .)