Idiots slip up outside bar (8)

'idiots' is the definition.
I know nothing about this answer so I cannot judge whether it can be defined by this definition.

'slip up outside bar' is the wordplay.
'up' is a reversal indicator (in down clue: letters go upwards).
'outside' is an insertion indicator (some letters go outside others).
'bar' becomes 'lock' (I've seen this in other clues).
'slip' in reverse letter order is 'pils'.
'pils' enclosing 'lock' is 'PILLOCKS'.

Can you help me to learn more?