A small quantity for one round thanks (4)
I believe the answer is:
iota
'a small quantity' is the definition.
(I've seen this in another clue)
'one round thanks' is the wordplay.
'one' becomes 'I' (Roman numeral).
'round' becomes 'o' (round can mean a round object).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'i'+'o'+'ta'='IOTA'
'for' is the link.
(Other definitions for iota that I've seen before include "Jot or tittle" , "Jot, particle" , "Tiny or scarcely detectable amount" , "letter from Greece" , "Ninthe letter of the Greek alphabet" .)