Organ observed filled with pulp, oddly (6)
I believe the answer is:
spleen
'organ' is the definition.
(the spleen is an organ)
'observed filled with pulp oddly' is the wordplay.
'observed' becomes 'seen' (synonyms).
'filled with' indicates putting letters inside.
'oddly' means one should take alternating letters (letters in the odd-numbered positions are used).
The alternate letters of 'pulp' are 'pl'.
'seen' enclosing 'pl' is 'SPLEEN'.
(Other definitions for spleen that I've seen before include "Bodily organ associated with bad humour" , "Bad temper from stomach organ" , "Acrimony" , "Organ causing bad temper?" , "Body organ; spite" .)