Something left out for head of ordinary delegation (8)
I believe the answer is:
omission
'something left out' is the definition.
(I've seen this in another clue)
'head of ordinary delegation' is the wordplay.
'head of' says to take the initial letters.
'delegation' becomes 'mission' (synonyms).
The first letter of 'ordinary' is 'o'.
'o'+'mission'='OMISSION'
'for' acts as a link.
(Other definitions for omission that I've seen before include "A thing left out" , "Leaving-out, oversight" , "It may create space" , "Lack" , "task not done" .)