Observe return of Eastern heavyweight (4)
I believe the answer is:
note
'observe' is the definition.
(I know that observe can be written as note)
'return of eastern heavyweight' is the wordplay.
'return of' says the letters should be written backwards.
'eastern' becomes 'e'.
'heavyweight' becomes 'ton' (a ton weighs a lot).
'e'+'ton'='eton'
'eton' backwards is 'NOTE'.
(Other definitions for note that I've seen before include "Observe, record" , "Bill" , "Crotchet or minim, perhaps" , "Fame - memorandum" , "Jotting" .)