Go wrong with foremost of SAS missions? (7)
I believe the answer is:
errands
'missions?' is the definition.
(I've seen this before)
'go wrong with foremost of sas' is the wordplay.
'go wrong' becomes 'err' (I've seen this in another clue).
'with' becomes 'and'.
'foremost of' says to take the initial letters.
The initial letter of 'sas' is 's'.
'err'+'and'+'s'='ERRANDS'
(Other definitions for errands that I've seen before include "Missions" , "Trips to carry out tasks" , "Short trips for messages" , "Chores" , "Tasks involving journeys" .)