Present US president's a cow (8)
I believe the answer is:
hereford
'a cow' is the definition.
(I've seen this before)
'present us president's' is the wordplay.
'present' becomes 'here' (both can mean in attendance).
'us president' becomes 'ford' (Gerald Ford).
'here'+'ford'='HEREFORD'
(Other definitions for hereford that I've seen before include "cross breed of cattle" , "City on the River Wye - cattle breed" , "Place" , "English cathedral city" , "Lower" .)