Exclaim about a bit of medicine and a blood vessel (9)
I believe the answer is:
capillary
'blood vessel' is the definition.
(capillary is a kind of blood vessel)
'exclaim about a bit of medicine and a' is the wordplay.
'exclaim' becomes 'cry' (I've seen this in another clue).
'about' means one lot of letters goes inside another.
'bit of medicine' becomes 'pill' (I've seen this in another clue).
'and' means one lot of letters go next to another.
'a'+'pill'+'a'='apilla'
'cry' enclosing 'apilla' is 'CAPILLARY'.
(Other definitions for capillary that I've seen before include "Minute blood vessel" , "Small internal diameter tube - a lyric pal (anag)" , "Small vessel" .)