Animal - one captured by British boy (5)
I believe the answer is:
bison
'animal' is the definition.
(I've seen this before)
'one captured by british boy' is the wordplay.
'one' becomes 'i' (Roman numeral).
'captured by' means one lot of letters goes inside another.
'british' becomes 'b' (abbreviation e.g. in BBC).
'boy' becomes 'son' (male child).
'b'+'son'='bson'
'i' placed inside 'bson' is 'BISON'.
(Other definitions for bison that I've seen before include "Large shaggy wild ox" , "Big beast" , "O bins could be full of buffalo" , "Buffalo Bill's prey" , "Shaggy-haired wild ox" .)