Ten, thanks, but a very small bit (4)
I believe the answer is:
iota
'a very small bit' is the definition.
(I've seen this in another clue)
'ten thanks' is the wordplay.
'ten' becomes 'io' (IO resembles 10).
'thanks' becomes 'ta' (UK informal version of 'thank you').
'io'+'ta'='IOTA'
'but' acts as a link.
(Other definitions for iota that I've seen before include "Smidgeon" , "Jot, particle" , "European letter" , "Tiny bit" , "The smallest possible amount, or the Greek i" .)