Try a daily worker in case of treatment (7)
I believe the answer is:
attempt
'try' is the definition.
(synonyms)
'a daily worker in case of treatment' is the wordplay.
'daily worker' becomes 'temp' (short-term or temporary worker).
'in' is an insertion indicator.
'case of' means to remove the middle letters (outsides of).
'treatment' with its middle removed is 'tt'.
'temp' going within 'tt' is 'ttempt'.
'a'+'ttempt'='ATTEMPT'
(Other definitions for attempt that I've seen before include "Stab" , "Try, make an effort" , "Try, endeavour" , "Tackle" , "Try, hazard" .)