Vent it to show anger (6)
I believe the answer is:
spleen
'vent it to show anger' is the definition.
(I've seen this before)
This is all the clue.
(Other definitions for spleen that I've seen before include "Bad temper from stomach organ" , "The organ once thought to be the seat of anger" , "Bodily organ associated with bad humour" , "Organ causing bad temper?" , "Quick temper - body organ" .)