Angry, extremely hostile sheep (3,2)

Here is my best explanation:

'angry' is the definition.
('het up' can be similar in meaning to 'angry')

'extremely hostile sheep' is the wordplay.
'extremely' suggests removing the centre.
'sheep' becomes 'tup' (tup is a kind of sheep).
'hostile' with its centre taken out is 'he'.
'he'+'tup'='HET UP'