British remain revolutionary in decline (3)
I believe the answer is:
ebb
'decline' is the definition.
(ebb is a kind of decline)
'british remain revolutionary in' is the wordplay.
'british' becomes 'b' (abbreviation e.g. in 'BBC').
'remain' becomes 'be' (I've seen this in other clues).
'revolutionary' is a reversal indicator.
'in' means one lot of letters goes inside another.
'be' written backwards gives 'eb'.
'b' placed inside 'eb' is 'EBB'.
(Other definitions for ebb that I've seen before include "Flow outwards" , "Type of tide" , "(Water) recede" , "Initially Mrs Browning's" , "Drain away; decline" .)