Decline eastern bishop twice (3)
I believe the answer is:
ebb
'decline' is the definition.
(ebb is a kind of decline)
'eastern bishop twice' is the wordplay.
'eastern' becomes 'e'.
'bishop' becomes 'b' (abbreviation used in chess).
'twice' indicates the doubling of some letters.
'b' duplicated is 'bb'.
'e'+'bb'='EBB'
(Other definitions for ebb that I've seen before include "Abate" , "Tidal outflow" , "state of decline" , "Flow outwards" , "Outward flow of the tide" .)