A locum tucking into teetotal shot (7)
I believe the answer is:
attempt
'shot' is the definition.
(shot can mean an attempt)
'a locum tucking into teetotal' is the wordplay.
'locum' becomes 'temp' (temporary worker).
'tucking into' is an insertion indicator.
'teetotal' becomes 'tt' (abbreviation).
'temp' inserted inside 'tt' is 'ttempt'.
'a'+'ttempt'='ATTEMPT'
(Other definitions for attempt that I've seen before include "Stab" , "Undertake" , "Try to climb (a peak)" , "Essay" , "bash" .)