Great man's expression of thanks in the box (5)
I believe the answer is:
titan
'great man's' is the definition.
(I've seen this before)
'expression of thanks in the box' is the wordplay.
'expression of thanks' becomes 'ta' (informal way of saying 'thank you').
'in' indicates putting letters inside.
'the box' becomes 'tin' (I've seen this in other clues).
'ta' put into 'tin' is 'TITAN'.
(Other definitions for titan that I've seen before include "Saturn's moon" , "Giant god of Gr. myth" , "distant moon" , "A son of Uranus and Gaea" , "Person of great size" .)