One member of team upset exercise group, I'm afraid (6)
I believe the answer is:
keeper
'one member of team' is the definition.
I can't judge whether this defines the answer.
'upset exercise group i'm afraid' is the wordplay.
'upset' is a reversal indicator.
'exercise group' becomes 'rep' (I am not sure about this - if you are sure you should believe this answer much more).
'i'm afraid' becomes 'eek' (expression of fear).
'rep'+'eek'='repeek'
'repeek' written backwards gives 'KEEPER'.
Can you help me to learn more?
(Other definitions for keeper that I've seen before include "Cain asked 'Am I my brother's ......?'" , "Football or cricket player" , "One at the wicket" , "Guardian, in the zoo say" , "Custodian; bar across magnet" .)