Falling off Big Ben (6)
I believe the answer is:
ebbing
'falling' is the definition.
(thesaurus)
'off big ben' is the wordplay.
'off' indicates anagramming the letters.
'big'+'ben'='bigben'
'bigben' is an anagram of 'EBBING'.
(Other definitions for ebbing that I've seen before include "Declining" , "Receding like Big Ben" , "Going out like a tide" , "in decline?" , "Gradually growing less" .)