Spotted little place inside that monitors blood supply (6)
I believe the answer is:
spleen
'that monitors blood supply' is the definition.
I can't tell whether this defines the answer.
'spotted little place inside' is the wordplay.
'spotted' becomes 'seen' (spot can mean to see or notice).
'little place' becomes 'pl' (abbreviation used in postal addresses. I am not sure about the 'little' bit.).
'inside' is an insertion indicator.
'seen' enclosing 'pl' is 'SPLEEN'.
Can you help me to learn more?
(Other definitions for spleen that I've seen before include "Internal organ; spite" , "Bitterness - body organ" , "Organ associated with ill humour" , "Peevishness, from bodily organ" , "The organ once thought to be the seat of anger" .)