Agent left, someone very good having had to be substituted (8)
I believe the answer is:
replaced
'be substituted' is the definition.
(I've seen this before)
'agent left someone very good having had' is the wordplay.
I cannot really see how this works, but
'agent' could be 'rep' (short for representative) and 'rep' is located in the answer.
'left' could be 'l' (common abbreviation) and 'l' is present in the answer.
'someone' could be 'ace' (ace is a kind of someone) and 'ace' is located in the answer.
'had' could be 'd' (in contractions e.g. 'I'd', 'you'd' etc) and 'd' is present in the answer.
This accounts for all the letters.
This may be the basis of the clue (or it may be nonsense).
'to' acts as a link.
Can you help me to learn more?
(Other definitions for replaced that I've seen before include "Substituted, exchanged" , "Led caper (anag.)" , "Substituted, supplanted" , "Put back in position" .)