Idiots cut up about senior teacher (8)

'idiots' is the definition.

'cut up about senior teacher' is the wordplay.
'cut' becomes 'snip' (snipping is a kind of cutting).
'up' is a reversal indicator (in down clue: letters go upwards).
'about' indicates putting letters inside.
'senior teacher' becomes 'head' (short for head teacher).
'snip' in reverse letter order is 'pins'.
'pins' enclosing 'head' is 'PINHEADS'.