Greek character led back to thanks (5)
I believe the answer is:
delta
'greek character' is the definition.
(letter from the Greek alphabet)
'led back to thanks' is the wordplay.
'back' shows that the letters should be reversed in order.
'to thanks' becomes 'ta' (UK informal version of 'thank you').
'led' written backwards gives 'del'.
'del'+'ta'='DELTA'
(Other definitions for delta that I've seen before include "Fourth Greek letter? - area at river mouth" , "Greek letter in the shape of a triangle" , "Nile river mouth" , "Fragmented estuary" , "Roughly triangular area of alluvial deposits" .)