Positive indications in last of many letters (6)
I believe the answer is:
'positive indications' is the definition.
(a yes is a positive answer)
'last of many letters' is the wordplay.
'last of' says to take the final letters.
'letters' becomes 'esses' (more than one S).
The last letter of 'many' is 'y'.
'in' acts as a link.
(Other definitions for yesses that I've seen before include "Votes in favour" , "Affirmatives" .)