Disconcerted athlete heading off, very embarrassed outwardly (8)
I believe the answer is:
unnerved
'disconcerted' is the definition.
(unnerving is a kind of disconcerting)
'athlete heading off very embarrassed outwardly' is the wordplay.
'athlete' becomes 'runner' (type of athlete).
'heading off' means to remove the first letter.
'very' becomes 'v' (abbreviation).
'embarrassed outwardly' becomes 'ed' (I can't explain this - if you can you should give a lot more credence to this answer).
'runner' with its first letter taken off is 'unner'.
'unner'+'v'+'ed'='UNNERVED'
Can you help me to learn more?
(Other definitions for unnerved that I've seen before include "Disconcerted" , "Caused to lose courage, shaken" , "Deprived of courage or confidence" , "Having lost courage or determination" , "Fazed, unsettled" .)