Small amount ultimately spent on publicity material (3)
I believe the answer is:
tad
'small amount' is the definition.
('a tad' is a small amount)
'ultimately spent on publicity material' is the wordplay.
'ultimately' says to take the final letters.
'on' means one lot of letters go next to another.
'publicity material' becomes 'ad' (short for advert).
The final letter of 'spent' is 't'.
't'+'ad'='TAD'
(Other definitions for tad that I've seen before include "Little bit" , "little boy" , "Smidgen" , "Small bit" , "Small amount or degree" .)