Tenner spent on curdling agent (6)
I believe the answer is:
rennet
'curdling agent' is the definition.
(I've seen this in another clue)
'tenner spent' is the wordplay.
'spent' indicates anagramming the letters.
'tenner' is an anagram of 'RENNET'.
'on' is the link.
(Other definitions for rennet that I've seen before include "cheesy setter?" , "Cheese-making curdler" , "apple" , "cheese maker?" , "Dairy compound" .)