King, when retiring, strokes bird (7)
I believe the answer is:
stephen
'king' is the definition.
(Stephen of Blois, King of England 1135-1154)
'retiring strokes bird' is the wordplay.
'retiring' says the letters should be written backwards.
'strokes' becomes 'pets' (I've seen this in another clue).
'bird' becomes 'hen' (hen is a kind of bird).
'pets' in reverse letter order is 'step'.
'step'+'hen'='STEPHEN'
'when' acts as a link.
(Other definitions for stephen that I've seen before include "Chap" , "English king from eleven hundred and thirty five" , "-- Fry; -- Hawking" , "Boxing Day saint" , "Author of The Shining" .)