Dan’s upset – by George! (4)
I believe the answer is:
sand
'george' is the definition.
(I've seen this before)
'dan's upset' is the wordplay.
'upset' indicates anagramming the letters.
'dans' is an anagram of 'SAND'.
'by' is the link.
(Other definitions for sand that I've seen before include "Yellow-brown colour" , "Parts of beach" , "woman writer Chopin loved" , "George -, Fr. author" , "Fine stone particles" .)