I'm grateful Sri Lankan hero has penned some yarns? (6,1,7)
I believe the answer is:
thanks a million
'i'm grateful' is the definition.
(phrase expressing gratitude)
'sri lankan hero has penned some yarns?' is the wordplay.
'sri lankan' becomes 'Tamil' (Tamil are a Sri Lankan ethnic group).
'hero' becomes 'lion'.
'has penned' is an insertion indicator (inserted letters are enclosed or penned in).
'some yarns?' becomes 'hanks' (hank can mean a roll of material).
'tamil'+'lion'='tamillion'
'tamillion' enclosing 'hanks' is 'THANKS A MILLION'.
(Other definitions for thanks a million that I've seen before include "that's very much appreciated! - with 18d" , "I appreciate that" , "(Often ironic) expression of gratitude" , "that's very much appreciated - with 8a" .)