Organ spotted outside public library (6)
I believe the answer is:
spleen
'organ' is the definition.
(I've seen this before)
'spotted outside public library' is the wordplay.
'spotted' becomes 'seen' (I've seen this before).
'outside' indicates putting letters inside.
'public library' becomes 'pl' (abbreviation for public library).
'seen' placed around 'pl' is 'SPLEEN'.
(Other definitions for spleen that I've seen before include "Bodily organ associated with bad temper" , "Organ supposedly controlling temper" , "Human organ" , "Internal organ; spite" , "Soft organ near the stomach" .)