Starts off dry and turns unusually mild, in fact (5)
I believe the answer is:
datum
'fact' is the definition.
(thesaurus)
'starts off dry and turns unusually mild' is the wordplay.
'starts off' indicates taking the first letters.
The initial letters of 'dry and turns unusually mild' is 'DATUM'.
'in' is the link.
(Other definitions for datum that I've seen before include "Bit of dope" , "Given piece of information" , "Little bit of information" , "Given item" , "Minimal information" .)