Peevishness observed about Poland (6)
I believe the answer is:
spleen
'peevishness' is the definition.
(I've seen this before)
'observed about poland' is the wordplay.
'observed' becomes 'seen' (synonyms).
'about' is an insertion indicator.
'poland' becomes 'PL' (e.g. on number plates).
'seen' going around 'pl' is 'SPLEEN'.
(Other definitions for spleen that I've seen before include "Soft organ near the stomach" , "Acrimony" , "Bad temper from stomach organ" , "Body organ; spite" , "Organ causing bad temper?" .)