Limits to some curious immunising agent (5)
I believe the answer is:
serum
'immunising agent' is the definition.
I can't tell whether this definition defines the answer.
'limits to some curious' is the wordplay.
'limits to' says to hollow out the word (remove centre letters) (only the edge letters are used).
'curious' becomes 'rum' (synonyms).
'some' with its centre removed is 'se'.
'se'+'rum'='SERUM'
Can you help me to learn more?
(Other definitions for serum that I've seen before include "Watery part of blood" , "Constituent of blood" , "Watery liquid as from blister, or fluid in blood" , "Blood component containing antibodies, used in immunisation" , "Amber-coloured fluid in the blood" .)