A little 'thank you' letter (3)
I believe the answer is:
tad
'a little' is the definition.
(I've seen this before)
'thank you letter' is the wordplay.
'thank you' becomes 'ta'.
'letter' becomes 'd' ().
'ta'+'d'='TAD'
(Other definitions for tad that I've seen before include "Modest amount" , "A small amount (colloq.)" , "Small bit" , "Little bit" , "Smidgen" .)