Move back; diminish (6)
I believe the answer is:
recede
I believe this clue is a double definition.
'move back' is the first definition.
(receding is a kind of moving back)
'diminish' is the second definition.
(thesaurus)
(Other definitions for recede that I've seen before include "Start balding" , "Decline" , "Withdraw" , "Go backwards" , "Go back, ebb" .)