At a function collecting trinketry (8)
I believe the answer is:
astatine
'at' is the definition.
(synonyms)
'a function collecting trinketry' is the wordplay.
'function' becomes 'sine' (mathematical function).
'collecting' indicates putting letters inside.
'trinketry' becomes 'tat' (I am not sure about this - if you are sure you should give a lot more credence to this answer).
'sine' placed around 'tat' is 'statine'.
'a'+'statine'='ASTATINE'
Can you help me to learn more?
(Other definitions for astatine that I've seen before include "Rare element" , "Chemical element At" , "radioactive material" , "Radioactive element" , "Atomic number 85" .)