Determine it in advance and impress Geordie trainer, essentially (9)
I believe the answer is:
preordain
'determine' is the definition.
(I know that preordain is a more specific form of predetermine)
'in advance and impress geordie trainer essentially' is the wordplay.
'in advance' becomes 'pre' (prefix meaning 'before').
'and' is a charade indicator (letters next to each other).
'impress geordie' becomes 'ord' (I can't justify this - if you can you should give a lot more credence to this answer).
'essentially' indicates the central letters.
The middle letters of 'trainer' are 'ain'.
'pre'+'ord'+'ain'='PREORDAIN'
'it' acts as a link.
This may not be right. It may belong to another bit of the clue.
Can you help me to learn more?
(Other definitions for preordain that I've seen before include "decree in advance" , "Determine early" , "set up well ahead" , "Decide beforehand" , "'Doom'" .)