Some worried over old animal (5)
I believe the answer is:
moose
'animal' is the definition.
(I've seen this before)
'some worried over old' is the wordplay.
'worried' indicates anagramming the letters.
'over' is an insertion indicator.
'old' becomes 'o' (common abbreviation eg in OE for Old English).
'some' with letters rearranged gives 'mose'.
'mose' enclosing 'o' is 'MOOSE'.
(Other definitions for moose that I've seen before include "Ungulate" , "beasts" , "Large US deer" , "US deer with enormous antlers" , "Large northern deer" .)