One British leader is a bird (4)
I believe the answer is:
ibis
'a bird' is the definition.
(ibis is a kind of bird)
'one british leader is' is the wordplay.
'one' becomes 'i' (Roman numeral).
'leader' suggests taking the first letters.
The initial letter of 'british' is 'b'.
'i'+'b'+'is'='IBIS'
(Other definitions for ibis that I've seen before include "Large wading bird" , "Retreat" , "Wading bird of warm regions" , "Hotel group" , "may be sacred?" .)