Pound locks up special animal (5)
I believe the answer is:
beast
'animal' is the definition.
(synonyms)
'pound locks up special' is the wordplay.
'pound' becomes 'beat' (synonyms).
'locks up' means one lot of letters goes inside another.
'special' becomes 'S' (common abbreviation as in Special Air Service).
'beat' going around 's' is 'BEAST'.
(Other definitions for beast that I've seen before include "Traditional poet" , "Nasty type" , "horse?" , "Animal, creature" , "wild creature" .)