An idiot standing back, not joining in (5)
I believe the answer is:
aloof
'back not joining in' is the definition.
I can't tell whether this defines the answer.
'an idiot standing' is the wordplay.
'an' becomes 'a'.
'idiot' becomes 'fool' (both can mean a stupid person).
'standing' is a reversal indicator (in a down clue, letters stand up).
'fool' in reverse letter order is 'loof'.
'a'+'loof'='ALOOF'
Can you help me to learn more?
(Other definitions for aloof that I've seen before include "Standoffish like a fool" , "Unfriendly or distant like a fool" , "Cool and remote, even unfriendly" , "Unpleasantly cold" , "Disinterested" .)