Ultimately, meat beat fish (5)
I believe the answer is:
trout
'fish' is the definition.
(trout is a kind of fish)
'ultimately meat beat' is the wordplay.
'ultimately' indicates one should take the final letters.
'beat' becomes 'rout' (routing is a kind of beating).
The last letter of 'meat' is 't'.
't'+'rout'='TROUT'
(Other definitions for trout that I've seen before include "Performance by five players" , "Unattractive old woman" , "one of a noted quintet" , "one may be hooked" , "Schubert work; fish" .)